Segment Information - Assets (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Segment reporting information
Total investments	$ 6,477.6	$ 6,388.7
Total assets	9,925.6	8,385.9
Total liabilities	4,794.6	3,824.3
Total White Mountains’s common shareholders’ equity	4,483.7	4,240.5
Noncontrolling interests	647.3	321.1
Debt	562.5	564.6
Subsidiary - Ark Insurance Holdings Limited
Segment reporting information
Noncontrolling interests	410.4	336.9
Subsidiary - HG Global
Segment reporting information
Noncontrolling interests	(13.4)	0.8
Kudu
Segment reporting information
Noncontrolling interests	127.6	113.8
Bamboo
Segment reporting information
Noncontrolling interests	113.6	0.0
Other
Segment reporting information
Noncontrolling interests	9.1	9.4
BAM Surplus Notes | Surplus Notes
Segment reporting information
Debt	300.9	322.2
Interest receivable	194.8	174.5
Other Operations
Segment reporting information
Total investments	1,570.4	1,709.1
Total assets	1,754.2	1,875.9
Total liabilities	181.3	141.1
Total White Mountains’s common shareholders’ equity	1,563.8	1,725.4
Noncontrolling interests	9.1	9.4
Debt	22.0	28.4
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Segment reporting information
Total investments	3,139.7	2,671.1
Total assets	5,299.0	4,378.6
Total liabilities	3,664.8	2,876.1
Total White Mountains’s common shareholders’ equity	1,223.8	1,165.6
Noncontrolling interests	410.4	336.9
Debt	154.5	185.5
Financial Guarantee (HG Global) | Operating Segments
Segment reporting information
Total investments	667.6	1,082.9
Total assets	1,179.4	1,172.0
Total liabilities	464.1	531.7
Total White Mountains’s common shareholders’ equity	728.7	779.3
Noncontrolling interests	(13.4)	(139.0)
Dividends payable	462.1	399.8
Debt	147.4	146.9
Asset Management (Kudu) | Operating Segments
Segment reporting information
Total investments	1,041.9	925.6
Total assets	1,108.4	959.4
Total liabilities	316.7	275.4
Total White Mountains’s common shareholders’ equity	664.1	570.2
Noncontrolling interests	127.6	113.8
Debt	238.6	203.8
P&C Insurance Distribution (Bamboo) | Operating Segments
Segment reporting information
Total investments	58.0	0.0
Total assets	584.6	0.0
Total liabilities	167.7	0.0
Total White Mountains’s common shareholders’ equity	303.3	0.0
Noncontrolling interests	$ 113.6	$ 0.0