Segment Information - Assets (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Segment Reporting Information [Line Items]
Total investments	$ 6,998.6	$ 6,417.6
Total assets	12,345.6	9,925.6
Total liabilities	6,628.5	4,794.6
Redeemable noncontrolling interests	132.4	0.0
Total White Mountains’s common shareholders’ equity	4,767.6	4,483.7
Nonredeemable noncontrolling interests	817.1	647.3
Other Operations
Segment Reporting Information [Line Items]
Total investments	1,260.2	1,568.4
Total assets	2,270.2	2,338.8
Total liabilities	556.3	349.0
Redeemable noncontrolling interests	0.0
Total White Mountains’s common shareholders’ equity	1,576.4	1,867.1
Nonredeemable noncontrolling interests	137.5	122.7
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Segment Reporting Information [Line Items]
Total investments	3,694.1	3,139.7
Total assets	6,796.8	5,299.0
Total liabilities	4,948.4	3,664.8
Redeemable noncontrolling interests	0.0
Total White Mountains’s common shareholders’ equity	1,367.7	1,223.8
Nonredeemable noncontrolling interests	480.7	410.4
Financial Guarantee (HG Global) | Operating Segments
Segment Reporting Information [Line Items]
Total investments	742.7	667.6
Total assets	1,266.1	1,179.4
Total liabilities	488.5	464.1
Redeemable noncontrolling interests	0.0
Total White Mountains’s common shareholders’ equity	789.5	728.7
Nonredeemable noncontrolling interests	(11.9)	(13.4)
Dividends payable	510.6	462.1
Asset Management (Kudu) | Operating Segments
Segment Reporting Information [Line Items]
Total investments	1,239.0	1,041.9
Total assets	1,298.8	1,108.4
Total liabilities	359.3	316.7
Redeemable noncontrolling interests	0.0
Total White Mountains’s common shareholders’ equity	804.0	664.1
Nonredeemable noncontrolling interests	135.5	127.6
Distinguished | Operating Segments
Segment Reporting Information [Line Items]
Total investments	62.6	0.0
Total assets	713.7	0.0
Total liabilities	276.0	0.0
Redeemable noncontrolling interests	132.4
Total White Mountains’s common shareholders’ equity	230.0	0.0
Nonredeemable noncontrolling interests	$ 75.3	$ 0.0