Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment reporting information
Earned insurance premiums	$ 1,440.9	$ 1,076.7	$ 664.2
Net investment income	194.2	124.4	82.5
Net investment income (expense) - BAM Surplus Note interest	0.0
Net realized and unrealized investment gains (losses)	434.2	(191.5)	(246.1)
Commission revenues	13.2	11.5	9.6
Other revenues	84.2	136.8	104.2
Total revenues	2,166.7	1,157.9	614.4
Loss and loss adjustment expenses	726.8	536.4	314.8
Acquisition expenses	290.1	250.6	186.3
Cost of sales	40.4	98.6	69.3
General and administrative expenses	432.6	364.1	291.6
Change in fair value of contingent consideration	48.7	17.3	5.5
Interest expense	62.7	40.3	20.5
Total expenses	1,601.3	1,307.3	888.0
Pre-tax income (loss) from continuing operations	565.4	(149.4)	(273.6)
Other Operations
Segment reporting information
Earned insurance premiums	0.0	0.0	0.0
Net investment income	30.1	32.2	18.2
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0
Commission revenues	13.2	11.5	9.6
Other revenues	80.5	127.2	90.7
Total revenues	339.4	76.3	(211.1)
Loss and loss adjustment expenses	0.0	0.0	0.0
Acquisition expenses	0.0	0.0	0.0
Cost of sales	40.4	98.6	69.3
General and administrative expenses	182.3	174.1	109.7
Change in fair value of contingent consideration	0.0	0.0	0.0
Interest expense	3.7	1.9	1.5
Total expenses	226.4	274.6	180.5
Pre-tax income (loss) from continuing operations	113.0	(198.3)	(391.6)
Investments, Excluding MediaAlpha
Segment reporting information
Net realized and unrealized investment gains (losses)	407.1	(98.5)	134.2
Investments, Excluding MediaAlpha | Other Operations
Segment reporting information
Net realized and unrealized investment gains (losses)	188.5	(1.6)	50.7
MediaAlpha
Segment reporting information
Net realized and unrealized investment gains (losses)	27.1	(93.0)	(380.3)
MediaAlpha | Other Operations
Segment reporting information
Net realized and unrealized investment gains (losses)	27.1	(93.0)	(380.3)
HG Global/BAM - Segment
Segment reporting information
Earned insurance premiums	31.2	33.3	26.9
HG Global/BAM - Segment | Operating Segments
Segment reporting information
Earned insurance premiums	31.2	33.3	26.9
Net investment income	31.7	21.5	17.5
Net realized and unrealized investment gains (losses)	26.6	(105.8)	(22.9)
Other revenues	2.9	4.6	1.5
Total revenues	92.4	(46.4)	23.0
Acquisition expenses	8.6	11.2	8.3
General and administrative expenses	68.9	69.1	57.1
Interest expense	16.5	8.3	0.0
Total expenses	94.0	88.6	65.4
HG Global/BAM - Segment | HG Global | Operating Segments
Segment reporting information
Earned insurance premiums	26.0	27.5	22.2
Net investment income	17.1	10.3	7.2
Net investment income (expense) - BAM Surplus Note interest	26.2	11.7	12.0
Commission revenues	0.0	0.0	0.0
Other revenues	0.0	0.5	0.5
Total revenues	82.9	(2.5)	28.2
Loss and loss adjustment expenses	0.0	0.0	0.0
Acquisition expenses	7.4	9.3	5.7
Cost of sales	0.0	0.0	0.0
General and administrative expenses	2.8	2.8	2.0
Change in fair value of contingent consideration	0.0	0.0	0.0
Interest expense	16.5	8.3	0.0
Total expenses	26.7	20.4	7.7
Pre-tax income (loss) from continuing operations	56.2	(22.9)	20.5
HG Global/BAM - Segment | HG Global | Investments, Excluding MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	13.6	(52.5)	(13.7)
HG Global/BAM - Segment | HG Global | MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	0.0	0.0	0.0
HG Global/BAM - Segment | BAM | Operating Segments
Segment reporting information
Earned insurance premiums	5.2	5.8	4.7
Net investment income	14.6	11.2	10.3
Net investment income (expense) - BAM Surplus Note interest	(26.2)	(11.7)	(12.0)
Commission revenues	0.0	0.0	0.0
Other revenues	2.9	4.1	1.0
Total revenues	9.5	(43.9)	(5.2)
Loss and loss adjustment expenses	0.0	0.0	0.0
Acquisition expenses	1.2	1.9	2.6
Cost of sales	0.0	0.0	0.0
General and administrative expenses	66.1	66.3	55.1
Change in fair value of contingent consideration	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0
Total expenses	67.3	68.2	57.7
Pre-tax income (loss) from continuing operations	(57.8)	(112.1)	(62.9)
HG Global/BAM - Segment | BAM | Investments, Excluding MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	13.0	(53.3)	(9.2)
HG Global/BAM - Segment | BAM | MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	0.0	0.0	0.0
Ark/WM Outrigger
Segment reporting information
Earned insurance premiums	1,409.7	1,043.4	637.3
Loss and loss adjustment expenses	726.8	536.4	314.8
Ark/WM Outrigger | Operating Segments
Segment reporting information
Earned insurance premiums	1,409.7	1,043.4	637.3
Net investment income	61.4	16.3	2.9
Net realized and unrealized investment gains (losses)	85.9	(55.2)	16.5
Other revenues	0.8	5.0	11.8
Total revenues	1,557.8	1,009.5	668.5
Loss and loss adjustment expenses	726.8	536.4	314.8
Acquisition expenses	281.5	239.4	178.0
General and administrative expenses	162.0	106.2	110.0
Change in fair value of contingent consideration	48.7	17.3	5.5
Interest expense	21.3	15.1	7.3
Total expenses	1,240.3	914.4	615.6
Ark/WM Outrigger | Ark | United Kingdom
Segment reporting information
Earned insurance premiums	791.7	638.5	459.3
Ark/WM Outrigger | Ark | Bermuda
Segment reporting information
Earned insurance premiums	513.7	404.9	178.0
Ark/WM Outrigger | Ark | Operating Segments
Segment reporting information
Earned insurance premiums	1,305.4	1,043.4	637.3
Net investment income	50.4	16.3	2.9
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0
Commission revenues	0.0	0.0	0.0
Other revenues	0.8	5.0	11.8
Total revenues	1,442.5	1,009.5	668.5
Loss and loss adjustment expenses	711.2	536.4	314.8
Acquisition expenses	251.0	239.4	178.0
Cost of sales	0.0	0.0	0.0
General and administrative expenses	161.7	106.2	110.0
Change in fair value of contingent consideration	48.7	17.3	5.5
Interest expense	21.3	15.1	7.3
Total expenses	1,193.9	914.4	615.6
Pre-tax income (loss) from continuing operations	248.6	95.1	52.9
Ark/WM Outrigger | Ark | Investments, Excluding MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	85.9	(55.2)	16.5
Ark/WM Outrigger | Ark | MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	0.0	0.0	0.0
Ark/WM Outrigger | WM Outrigger Re | Operating Segments
Segment reporting information
Earned insurance premiums	104.3
Net investment income	11.0
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0
Commission revenues	0.0
Other revenues	0.0
Total revenues	115.3
Loss and loss adjustment expenses	15.6
Acquisition expenses	30.5
Cost of sales	0.0
General and administrative expenses	0.3
Change in fair value of contingent consideration	0.0
Interest expense	0.0
Total expenses	46.4
Pre-tax income (loss) from continuing operations	68.9
Ark/WM Outrigger | WM Outrigger Re | Investments, Excluding MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	0.0
Ark/WM Outrigger | WM Outrigger Re | MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	0.0
Kudu | Operating Segments
Segment reporting information
Earned insurance premiums	0.0	0.0	0.0
Net investment income	71.0	54.4	43.9
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	106.1	64.1	89.9
Commission revenues	0.0	0.0	0.0
Other revenues	0.0	0.0	0.2
Total revenues	177.1	118.5	134.0
Loss and loss adjustment expenses	0.0	0.0	0.0
Acquisition expenses	0.0	0.0	0.0
Cost of sales	0.0	0.0	0.0
General and administrative expenses	19.4	14.7	14.8
Change in fair value of contingent consideration	0.0	0.0	0.0
Interest expense	21.2	15.0	11.7
Total expenses	40.6	29.7	26.5
Pre-tax income (loss) from continuing operations	136.5	88.8	107.5
Kudu | Investments, Excluding MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	106.1	64.1	89.9
Kudu | MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)	$ 0.0	$ 0.0	$ 0.0