Segment Information - Assets (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Segment reporting information
Total investments	$ 6,388.7	$ 5,170.0
Total assets	8,385.9	7,389.3
Total liabilities	3,824.3	3,454.3
Total White Mountains’s common shareholders’ equity	4,240.5	3,746.9
Noncontrolling interests	321.1	188.1
Debt	564.6	575.2
BAM Surplus Notes | Surplus Notes
Segment reporting information
Debt	322.2	340.0
Interest receivable	174.5	157.9
Dividends payable	399.8	341.4
Other Operations
Segment reporting information
Total investments	1,709.1	1,532.7
Total assets	1,875.9	1,813.5
Total liabilities	141.1	157.1
Total White Mountains’s common shareholders’ equity	1,725.4	1,636.0
Noncontrolling interests	9.4	20.4
Debt	28.4	36.7
HG Global/BAM - Segment | Operating Segments
Segment reporting information
Total investments	1,082.9	975.8
Total assets	1,172.0	1,058.5
Total liabilities	531.7	501.8
Total White Mountains’s common shareholders’ equity	779.3	712.0
Noncontrolling interests	(139.0)	(155.3)
Debt	146.9	146.5
Ark | Operating Segments
Segment reporting information
Total investments	2,671.1	1,965.6
Total assets	4,378.6	3,691.4
Total liabilities	2,876.1	2,522.1
Total White Mountains’s common shareholders’ equity	1,165.6	921.4
Noncontrolling interests	336.9	247.9
Debt	185.5	183.7
Kudu | Operating Segments
Segment reporting information
Total investments	925.6	695.9
Total assets	959.4	825.9
Total liabilities	275.4	273.3
Total White Mountains’s common shareholders’ equity	570.2	477.5
Noncontrolling interests	113.8	75.1
Debt	$ 203.8	$ 208.3