Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Earned insurance premiums	$ 202.8	$ 111.0
Net investment income	20.6	20.6
Net realized and unrealized investment gains (losses)	(8.4)	1.1
Commission revenues	73.0	61.9
Other revenues	42.1	25.3
Total revenues	348.9	178.2
Loss and loss adjustment expenses	122.0	66.0
Insurance and reinsurance acquisition expenses	52.9	38.6
Cost of sales	21.4	4.0
General and administrative expenses	125.0	138.4
Broker commission expenses	20.6	18.9
Change in fair value of contingent consideration	0.1
Amortization of other intangible assets	10.1	9.2
Loss on assets held for sale	0.0	28.7
Interest expense	9.0	13.1
Total expenses	361.1	316.9
Pre-tax income (loss) from continuing operations	(12.2)	(138.7)
Marketing Technology (MediaAlpha)
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	18.8	(41.7)
Financial Guarantee (HG Global/BAM)
Segment Reporting Information [Line Items]
Earned insurance premiums	8.4	6.4
Net investment income	4.6	4.5
Net realized and unrealized investment gains (losses)	(45.1)	(17.9)
Commission revenues	0.0	0.0
Other revenues	0.8	0.3
Total revenues	(31.3)	(6.7)
Loss and loss adjustment expenses	0.0	0.0
Insurance and reinsurance acquisition expenses	3.0	1.9
Cost of sales	0.0	0.0
General and administrative expenses	16.3	16.4
Broker commission expenses	0.0	0.0
Change in fair value of contingent consideration	0.0
Amortization of other intangible assets	0.0	0.0
Loss on assets held for sale	0.0	0.0
Interest expense	0.0	0.0
Total expenses	19.3	18.3
Pre-tax income (loss) from continuing operations	(50.6)	(25.0)
Financial Guarantee (HG Global/BAM) | Marketing Technology (MediaAlpha)
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	0.0	0.0
Ark
Segment Reporting Information [Line Items]
Earned insurance premiums	194.4	104.6
Net investment income	1.6	0.8
Net realized and unrealized investment gains (losses)	(17.5)	1.1
Commission revenues	0.0	0.0
Other revenues	(2.8)	2.6
Total revenues	175.7	109.1
Loss and loss adjustment expenses	122.0	66.0
Insurance and reinsurance acquisition expenses	49.9	36.7
Cost of sales	0.0	0.0
General and administrative expenses	23.1	37.8
Broker commission expenses	0.0	0.0
Change in fair value of contingent consideration	0.0
Amortization of other intangible assets	0.0	0.0
Loss on assets held for sale	0.0	0.0
Interest expense	3.8	1.1
Total expenses	198.8	141.6
Pre-tax income (loss) from continuing operations	(23.1)	(32.5)
Ark | Marketing Technology (MediaAlpha)
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	0.0	0.0
NSM Insurance Group [Member]
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0
Net investment income	0.0	0.0
Net realized and unrealized investment gains (losses)	0.0	0.0
Commission revenues	70.1	59.6
Other revenues	18.4	15.2
Total revenues	88.5	74.8
Loss and loss adjustment expenses	0.0	0.0
Insurance and reinsurance acquisition expenses	0.0	0.0
Cost of sales	0.0	0.0
General and administrative expenses	54.0	46.0
Broker commission expenses	20.6	18.9
Change in fair value of contingent consideration	0.1	0.0
Amortization of other intangible assets	9.1	8.6
Loss on assets held for sale	0.0	28.7
Interest expense	2.1	5.9
Total expenses	85.9	108.1
Pre-tax income (loss) from continuing operations	2.6	(33.3)
NSM Insurance Group [Member] | Marketing Technology (MediaAlpha)
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	0.0	0.0
Asset Management (Kudu)
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0
Net investment income	12.6	8.2
Net realized and unrealized investment gains (losses)	22.3	15.8
Commission revenues	0.0	0.0
Other revenues	0.0	0.1
Total revenues	34.9	24.1
Loss and loss adjustment expenses	0.0	0.0
Insurance and reinsurance acquisition expenses	0.0	0.0
Cost of sales	0.0	0.0
General and administrative expenses	2.7	2.5
Broker commission expenses	0.0	0.0
Change in fair value of contingent consideration	0.0
Amortization of other intangible assets	0.1	0.1
Loss on assets held for sale	0.0	0.0
Interest expense	2.8	5.8
Total expenses	5.6	8.4
Pre-tax income (loss) from continuing operations	29.3	15.7
Asset Management (Kudu) | Marketing Technology (MediaAlpha)
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	0.0	0.0
Other Operations
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0
Net investment income	1.8	7.1
Net realized and unrealized investment gains (losses)	31.9	2.1
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	18.8	(41.7)
Commission revenues	2.9	2.3
Other revenues	25.7	7.1
Total revenues	81.1	(23.1)
Loss and loss adjustment expenses	0.0	0.0
Insurance and reinsurance acquisition expenses	0.0	0.0
Cost of sales	21.4	4.0
General and administrative expenses	28.9	35.7
Broker commission expenses	0.0	0.0
Change in fair value of contingent consideration	0.0
Amortization of other intangible assets	0.9	0.5
Loss on assets held for sale	0.0	0.0
Interest expense	0.3	0.3
Total expenses	51.5	40.5
Pre-tax income (loss) from continuing operations	$ 29.6	$ (63.6)