Segment Information - Assets (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2012
Segment reporting information
Total investments	$ 4,265.3	$ 2,939.0
Total assets	7,000.7	4,831.4
Total liabilities	3,296.0	1,013.5
Total White Mountains’s common shareholders’ equity	3,548.1	3,906.0
Non-controlling interests	156.6	(88.1)
Held for Sale
Segment reporting information
Total investments	0.0	0.0
Total assets	16.1	2.3
Total liabilities	0.0	0.0
Total White Mountains’s common shareholders’ equity	16.1	2.3
Non-controlling interests	0.0	0.0
HG Global-BAM
Segment reporting information
Total investments	966.5	919.9
Total assets	1,044.8	1,017.8
Total liabilities	321.9	291.5
Total White Mountains’s common shareholders’ equity	838.0	836.1
Non-controlling interests	(115.1)	(109.8)
Ark
Segment reporting information
Total investments	1,562.1	0.0
Total assets	3,027.0	0.0
Total liabilities	2,122.4	0.0
Total White Mountains’s common shareholders’ equity	673.9	0.0
Non-controlling interests	230.7	0.0
NSM
Segment reporting information
Total investments	0.0	0.0
Total assets	977.3	999.6
Total liabilities	495.3	491.8
Total White Mountains’s common shareholders’ equity	465.3	490.7
Non-controlling interests	16.7	17.1
Kudu
Segment reporting information
Total investments	669.5	400.6
Total assets	727.1	430.2
Total liabilities	261.0	96.3
Total White Mountains’s common shareholders’ equity	453.7	331.6
Non-controlling interests	12.4	2.3
Other Operations
Segment reporting information
Total investments	1,067.2	1,618.5
Total assets	1,208.4	2,381.5
Total liabilities	95.4	133.9
Total White Mountains’s common shareholders’ equity	1,101.1	2,245.3
Non-controlling interests	11.9	2.3
BAM
Segment reporting information
Surplus notes	364.6	388.2
Accrued interest on surplus notes	157.6	155.7
HG Global
Segment reporting information
Non-controlling interests	8.9	13.5
Surplus notes			$ 503.0
Preferred Dividends Payable | HG Global
Segment reporting information
Total liabilities	$ 400.5	$ 363.9