Segment Information - Assets (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2012
Segment reporting information
Total investments	$ 5,170.0	$ 4,257.5
Total assets	7,389.3	7,000.7
Total liabilities	3,454.3	3,296.0
Total White Mountains’s common shareholders’ equity	3,746.9	3,548.1
Non-controlling interests	188.1	156.6
Held for Sale
Segment reporting information
Total investments	0.0	0.0
Total assets	0.0	1,005.1
Total liabilities	0.0	495.3
Total White Mountains’s common shareholders’ equity	0.0	493.1
Non-controlling interests	0.0	16.7
HG Global-BAM
Segment reporting information
Total investments	975.8	966.5
Total assets	1,058.5	1,044.8
Total liabilities	501.8	321.9
Total White Mountains’s common shareholders’ equity	712.0	838.0
Non-controlling interests	(155.3)	(115.1)
Ark
Segment reporting information
Total investments	1,761.9	1,562.1
Total assets	3,486.2	3,027.0
Total liabilities	2,520.9	2,122.4
Total White Mountains’s common shareholders’ equity	717.4	673.9
Non-controlling interests	247.9	230.7
Kudu
Segment reporting information
Total investments	695.9	669.5
Total assets	825.9	727.1
Total liabilities	273.3	261.0
Total White Mountains’s common shareholders’ equity	477.5	453.7
Non-controlling interests	75.1	12.4
Other Operations
Segment reporting information
Total investments	1,736.4	1,059.4
Total assets	2,018.7	1,196.7
Total liabilities	158.3	95.4
Total White Mountains’s common shareholders’ equity	1,840.0	1,089.4
Non-controlling interests	20.4	11.9
BAM
Segment reporting information
Surplus notes	340.0	364.6
Accrued interest on surplus notes	157.9	157.6
HG Global
Segment reporting information
Non-controlling interests	(0.6)	8.9
Surplus notes			$ 503.0
Preferred Dividends Payable | HG Global
Segment reporting information
Total liabilities	$ 341.4	$ 400.5