{"cik":776867,"taxonomy":"us-gaap","tag":"EarningsPerShareBasicAndDiluted","label":"Earnings Per Share, Basic and Diluted","description":"The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.","entityName":"WHITE MOUNTAINS INSURANCE GROUP, LTD","units":{"USD/shares":[{"start":"2015-01-01","end":"2015-12-31","val":27.22,"accn":"0000776867-18-000002","fy":2017,"fp":"FY","form":"10-K","filed":"2018-02-28","frame":"CY2015"},{"start":"2016-01-01","end":"2016-12-31","val":-24.26,"accn":"0000776867-18-000002","fy":2017,"fp":"FY","form":"10-K","filed":"2018-02-28"},{"start":"2016-01-01","end":"2016-12-31","val":-24.26,"accn":"0000776867-19-000008","fy":2018,"fp":"FY","form":"10-K","filed":"2019-02-27","frame":"CY2016"},{"start":"2017-01-01","end":"2017-03-31","val":-0.52,"accn":"0000776867-18-000006","fy":2018,"fp":"Q1","form":"10-Q","filed":"2018-05-02","frame":"CY2017Q1"},{"start":"2017-01-01","end":"2017-06-30","val":1.84,"accn":"0000776867-18-000018","fy":2018,"fp":"Q2","form":"10-Q","filed":"2018-08-07"},{"start":"2017-04-01","end":"2017-06-30","val":2.36,"accn":"0000776867-18-000018","fy":2018,"fp":"Q2","form":"10-Q","filed":"2018-08-07","frame":"CY2017Q2"},{"start":"2017-01-01","end":"2017-09-30","val":7.03,"accn":"0000776867-18-000023","fy":2018,"fp":"Q3","form":"10-Q","filed":"2018-11-05"},{"start":"2017-07-01","end":"2017-09-30","val":5.36,"accn":"0000776867-18-000023","fy":2018,"fp":"Q3","form":"10-Q","filed":"2018-11-05","frame":"CY2017Q3"},{"start":"2017-01-01","end":"2017-12-31","val":11.56,"accn":"0000776867-18-000002","fy":2017,"fp":"FY","form":"10-K","filed":"2018-02-28"},{"start":"2017-01-01","end":"2017-12-31","val":11.56,"accn":"0000776867-19-000008","fy":2018,"fp":"FY","form":"10-K","filed":"2019-02-27"},{"start":"2017-01-01","end":"2017-12-31","val":11.56,"accn":"0000776867-20-000009","fy":2019,"fp":"FY","form":"10-K","filed":"2020-03-02","frame":"CY2017"},{"start":"2018-01-01","end":"2018-03-31","val":-12.85,"accn":"0000776867-18-000006","fy":2018,"fp":"Q1","form":"10-Q","filed":"2018-05-02"},{"start":"2018-01-01","end":"2018-03-31","val":-12.85,"accn":"0000776867-19-000015","fy":2019,"fp":"Q1","form":"10-Q","filed":"2019-05-06","frame":"CY2018Q1"},{"start":"2018-01-01","end":"2018-06-30","val":-12.44,"accn":"0000776867-18-000018","fy":2018,"fp":"Q2","form":"10-Q","filed":"2018-08-07"},{"start":"2018-01-01","end":"2018-06-30","val":-12.44,"accn":"0000776867-19-000024","fy":2019,"fp":"Q2","form":"10-Q","filed":"2019-08-06"},{"start":"2018-04-01","end":"2018-06-30","val":1.02,"accn":"0000776867-18-000018","fy":2018,"fp":"Q2","form":"10-Q","filed":"2018-08-07"},{"start":"2018-04-01","end":"2018-06-30","val":1.02,"accn":"0000776867-19-000024","fy":2019,"fp":"Q2","form":"10-Q","filed":"2019-08-06","frame":"CY2018Q2"},{"start":"2018-01-01","end":"2018-09-30","val":3.9,"accn":"0000776867-18-000023","fy":2018,"fp":"Q3","form":"10-Q","filed":"2018-11-05"},{"start":"2018-01-01","end":"2018-09-30","val":3.9,"accn":"0000776867-19-000030","fy":2019,"fp":"Q3","form":"10-Q","filed":"2019-11-06"},{"start":"2018-07-01","end":"2018-09-30","val":18.27,"accn":"0000776867-18-000023","fy":2018,"fp":"Q3","form":"10-Q","filed":"2018-11-05"},{"start":"2018-07-01","end":"2018-09-30","val":18.27,"accn":"0000776867-19-000030","fy":2019,"fp":"Q3","form":"10-Q","filed":"2019-11-06","frame":"CY2018Q3"},{"start":"2018-01-01","end":"2018-12-31","val":-36.67,"accn":"0000776867-19-000008","fy":2018,"fp":"FY","form":"10-K","filed":"2019-02-27"},{"start":"2018-01-01","end":"2018-12-31","val":-36.67,"accn":"0000776867-20-000009","fy":2019,"fp":"FY","form":"10-K","filed":"2020-03-02"},{"start":"2018-01-01","end":"2018-12-31","val":-36.67,"accn":"0000776867-21-000004","fy":2020,"fp":"FY","form":"10-K","filed":"2021-02-26","frame":"CY2018"},{"start":"2019-01-01","end":"2019-03-31","val":89.42,"accn":"0000776867-19-000015","fy":2019,"fp":"Q1","form":"10-Q","filed":"2019-05-06"},{"start":"2019-01-01","end":"2019-03-31","val":89.42,"accn":"0000776867-20-000016","fy":2019,"fp":"Q3","form":"10-Q","filed":"2020-05-08","frame":"CY2019Q1"},{"start":"2019-01-01","end":"2019-06-30","val":95.71,"accn":"0000776867-19-000024","fy":2019,"fp":"Q2","form":"10-Q","filed":"2019-08-06"},{"start":"2019-01-01","end":"2019-06-30","val":95.71,"accn":"0000776867-20-000031","fy":2019,"fp":"Q2","form":"10-Q","filed":"2020-08-07"},{"start":"2019-04-01","end":"2019-06-30","val":6.44,"accn":"0000776867-19-000024","fy":2019,"fp":"Q2","form":"10-Q","filed":"2019-08-06"},{"start":"2019-04-01","end":"2019-06-30","val":6.44,"accn":"0000776867-20-000031","fy":2019,"fp":"Q2","form":"10-Q","filed":"2020-08-07","frame":"CY2019Q2"},{"start":"2019-01-01","end":"2019-09-30","val":110.67,"accn":"0000776867-19-000030","fy":2019,"fp":"Q3","form":"10-Q","filed":"2019-11-06"},{"start":"2019-01-01","end":"2019-09-30","val":110.67,"accn":"0000776867-20-000036","fy":2020,"fp":"Q3","form":"10-Q","filed":"2020-11-06"},{"start":"2019-07-01","end":"2019-09-30","val":15.01,"accn":"0000776867-19-000030","fy":2019,"fp":"Q3","form":"10-Q","filed":"2019-11-06"},{"start":"2019-07-01","end":"2019-09-30","val":15.01,"accn":"0000776867-20-000036","fy":2020,"fp":"Q3","form":"10-Q","filed":"2020-11-06","frame":"CY2019Q3"},{"start":"2019-01-01","end":"2019-12-31","val":130.02,"accn":"0000776867-20-000009","fy":2019,"fp":"FY","form":"10-K","filed":"2020-03-02"},{"start":"2019-01-01","end":"2019-12-31","val":130.02,"accn":"0000776867-21-000004","fy":2020,"fp":"FY","form":"10-K","filed":"2021-02-26"},{"start":"2019-01-01","end":"2019-12-31","val":130.02,"accn":"0000776867-22-000002","fy":2021,"fp":"FY","form":"10-K","filed":"2022-02-28","frame":"CY2019"},{"start":"2020-01-01","end":"2020-03-31","val":-40.82,"accn":"0000776867-20-000016","fy":2019,"fp":"Q3","form":"10-Q","filed":"2020-05-08"},{"start":"2020-01-01","end":"2020-03-31","val":-40.82,"accn":"0000776867-21-000008","fy":2021,"fp":"Q1","form":"10-Q","filed":"2021-05-10","frame":"CY2020Q1"},{"start":"2020-01-01","end":"2020-06-30","val":-4.25,"accn":"0000776867-20-000031","fy":2019,"fp":"Q2","form":"10-Q","filed":"2020-08-07"},{"start":"2020-01-01","end":"2020-06-30","val":-4.25,"accn":"0000776867-21-000016","fy":2021,"fp":"Q2","form":"10-Q","filed":"2021-08-06"},{"start":"2020-04-01","end":"2020-06-30","val":37.46,"accn":"0000776867-20-000031","fy":2019,"fp":"Q2","form":"10-Q","filed":"2020-08-07"},{"start":"2020-04-01","end":"2020-06-30","val":37.46,"accn":"0000776867-21-000016","fy":2021,"fp":"Q2","form":"10-Q","filed":"2021-08-06","frame":"CY2020Q2"},{"start":"2020-01-01","end":"2020-09-30","val":70.4,"accn":"0000776867-20-000036","fy":2020,"fp":"Q3","form":"10-Q","filed":"2020-11-06"},{"start":"2020-01-01","end":"2020-09-30","val":70.4,"accn":"0000776867-21-000021","fy":2021,"fp":"Q3","form":"10-Q","filed":"2021-11-08"},{"start":"2020-07-01","end":"2020-09-30","val":75.32,"accn":"0000776867-20-000036","fy":2020,"fp":"Q3","form":"10-Q","filed":"2020-11-06"},{"start":"2020-07-01","end":"2020-09-30","val":75.32,"accn":"0000776867-21-000021","fy":2021,"fp":"Q3","form":"10-Q","filed":"2021-11-08","frame":"CY2020Q3"},{"start":"2020-01-01","end":"2020-12-31","val":227.72,"accn":"0000776867-21-000004","fy":2020,"fp":"FY","form":"10-K","filed":"2021-02-26"},{"start":"2020-01-01","end":"2020-12-31","val":227.72,"accn":"0000776867-22-000002","fy":2021,"fp":"FY","form":"10-K","filed":"2022-02-28","frame":"CY2020"},{"start":"2021-01-01","end":"2021-03-31","val":-30.33,"accn":"0000776867-21-000008","fy":2021,"fp":"Q1","form":"10-Q","filed":"2021-05-10"},{"start":"2021-01-01","end":"2021-03-31","val":-30.33,"accn":"0000776867-22-000005","fy":2022,"fp":"Q1","form":"10-Q","filed":"2022-05-06","frame":"CY2021Q1"},{"start":"2021-01-01","end":"2021-06-30","val":14.32,"accn":"0000776867-21-000016","fy":2021,"fp":"Q2","form":"10-Q","filed":"2021-08-06"},{"start":"2021-04-01","end":"2021-06-30","val":44.56,"accn":"0000776867-21-000016","fy":2021,"fp":"Q2","form":"10-Q","filed":"2021-08-06","frame":"CY2021Q2"},{"start":"2021-01-01","end":"2021-09-30","val":-105.48,"accn":"0000776867-21-000021","fy":2021,"fp":"Q3","form":"10-Q","filed":"2021-11-08"},{"start":"2021-07-01","end":"2021-09-30","val":-120.18,"accn":"0000776867-21-000021","fy":2021,"fp":"Q3","form":"10-Q","filed":"2021-11-08","frame":"CY2021Q3"},{"start":"2021-01-01","end":"2021-12-31","val":-95.53,"accn":"0000776867-22-000002","fy":2021,"fp":"FY","form":"10-K","filed":"2022-02-28","frame":"CY2021"},{"start":"2022-01-01","end":"2022-03-31","val":11.1,"accn":"0000776867-22-000005","fy":2022,"fp":"Q1","form":"10-Q","filed":"2022-05-06","frame":"CY2022Q1"}]}}