Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Earned insurance premiums	$ 6.2	$ 5.2	$ 17.2	$ 13.7
Net investment income	71.2	18.4	113.7	55.4
Net realized and unrealized investment gains	306.6	67.0	302.8	233.1
Realized gain and unrealized investment gain from the MediaAlpha Transaction				182.2
Advertising and commission revenues	60.3	51.3	180.3	200.9
Other revenues	15.2	13.6	45.9	32.8
Total revenues	459.5	155.5	659.9	718.1
Insurance acquisition expenses	1.6	1.4	5.4	4.1
Other underwriting expenses	0.1	0.1	0.3	0.3
Cost of sales	2.3	2.3	6.5	45.6
General and administrative expenses	103.3	78.0	266.7	236.9
Broker commission expenses	17.1	15.3	56.4	48.9
Change in fair value of contingent consideration earnout liabilities	0.7	(2.0)	(1.6)	5.6
Amortization of other intangible assets	5.4	4.6	17.1	15.4
Interest expense	7.8	4.7	21.2	12.7
Total expenses	138.3	104.4	372.0	369.5
Pre-tax income (loss) from continuing operations	321.2	51.1	287.9	348.6
HG Global/ BAM
Segment Reporting Information [Line Items]
Earned insurance premiums	6.2	5.2	17.2	13.7
Net investment income	4.7	5.4	15.1	16.1
Net realized and unrealized investment gains	3.2	7.1	23.7	30.4
Realized gain and unrealized investment gain from the MediaAlpha Transaction				0.0
Advertising and commission revenues	0.0	0.0	0.0	0.0
Other revenues	0.4	0.3	2.1	1.3
Total revenues	14.5	18.0	58.1	61.5
Insurance acquisition expenses	1.6	1.4	5.4	4.1
Other underwriting expenses	0.1	0.1	0.3	0.3
Cost of sales	0.0	0.0	0.0	0.0
General and administrative expenses	13.9	12.5	41.1	39.1
Broker commission expenses	0.0	0.0	0.0	0.0
Change in fair value of contingent consideration earnout liabilities	0.0	0.0	0.0	0.0
Amortization of other intangible assets	0.0	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	0.0
Total expenses	15.6	14.0	46.8	43.5
Pre-tax income (loss) from continuing operations	(1.1)	4.0	11.3	18.0
NSM
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0	0.0	0.0
Net investment income	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains	0.0	0.0	0.0	0.0
Realized gain and unrealized investment gain from the MediaAlpha Transaction				0.0
Advertising and commission revenues	58.2	49.4	174.2	147.6
Other revenues	12.5	10.5	37.6	27.7
Total revenues	70.7	59.9	211.8	175.3
Insurance acquisition expenses	0.0	0.0		0.0
Other underwriting expenses	0.0	0.0	0.0	0.0
Cost of sales	0.0	0.0	0.0	0.0
General and administrative expenses	42.9	33.3	131.0	91.4
Broker commission expenses	17.1	15.3	56.4	48.9
Change in fair value of contingent consideration earnout liabilities	0.7	(2.0)	(1.6)	5.6
Amortization of other intangible assets	5.1	4.4	16.2	13.5
Interest expense	6.1	4.4	16.1	12.2
Total expenses	71.9	55.4	218.1	171.6
Pre-tax income (loss) from continuing operations	(1.2)	4.5	(6.3)	3.7
Kudu
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0	0.0	0.0
Net investment income	6.4	4.6	19.3	8.6
Net realized and unrealized investment gains	9.8	2.7	1.5	3.1
Realized gain and unrealized investment gain from the MediaAlpha Transaction				0.0
Advertising and commission revenues	0.0	0.0	0.0	0.0
Other revenues	0.1	0.2	0.2	0.2
Total revenues	16.3	7.5	21.0	11.9
Insurance acquisition expenses	0.0	0.0	0.0	0.0
Other underwriting expenses	0.0	0.0	0.0	0.0
Cost of sales	0.0	0.0	0.0	0.0
General and administrative expenses	2.2	4.5	7.5	6.7
Broker commission expenses	0.0	0.0	0.0	0.0
Change in fair value of contingent consideration earnout liabilities	0.0	0.0	0.0	0.0
Amortization of other intangible assets	0.1	0.2	0.3	0.2
Interest expense	1.4	0.0	4.3	0.0
Total expenses	3.7	4.7	12.1	6.9
Pre-tax income (loss) from continuing operations	12.6	2.8	8.9	5.0
MediaAlpha
Segment Reporting Information [Line Items]
Earned insurance premiums				0.0
Net investment income				0.0
Net realized and unrealized investment gains				0.0
Realized gain and unrealized investment gain from the MediaAlpha Transaction				0.0
Advertising and commission revenues	0.0	0.0	0.0	48.8
Other revenues				0.0
Total revenues	0.0	0.0	0.0	48.8
Insurance acquisition expenses				0.0
Other underwriting expenses				0.0
Cost of sales	0.0	0.0	0.0	40.6
General and administrative expenses	0.0	0.0	0.0	12.5
Broker commission expenses				0.0
Change in fair value of contingent consideration earnout liabilities				0.0
Amortization of other intangible assets	0.0	0.0	0.0	1.6
Interest expense	0.0	0.0	0.0	0.2
Total expenses	0.0	0.0	0.0	54.9
Pre-tax income (loss) from continuing operations				(6.1)
Other Operations
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0	0.0	0.0
Net investment income	60.1	8.4	79.3	30.7
Net realized and unrealized investment gains	293.6	57.2	277.6	199.6
Realized gain and unrealized investment gain from the MediaAlpha Transaction	0.0	0.0	0.0	182.2
Advertising and commission revenues	2.1	1.9	6.1	4.5
Other revenues	2.2	2.6	6.0	3.6
Total revenues	358.0	70.1	369.0	420.6
Insurance acquisition expenses	0.0	0.0	0.0	0.0
Other underwriting expenses	0.0	0.0	0.0	0.0
Cost of sales	2.3	2.3	6.5	5.0
General and administrative expenses	44.3	27.7	87.1	87.2
Broker commission expenses	0.0	0.0	0.0	0.0
Change in fair value of contingent consideration earnout liabilities	0.0	0.0	0.0	0.0
Amortization of other intangible assets	0.2	0.0	0.6	0.1
Interest expense	0.3	0.3	0.8	0.3
Total expenses	47.1	30.3	95.0	92.6
Pre-tax income (loss) from continuing operations	$ 310.9	$ 39.8	$ 274.0	$ 328.0