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CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($)		3 Months Ended								8 Months Ended	9 Months Ended	10 Months Ended	12 Months Ended
	Sep. 28, 2017	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018	Sep. 28, 2017	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues
Earned insurance premiums													$ 16,300,000	$ 13,900,000	$ 10,400,000
Net investment income													79,700,000	59,000,000.0	56,000,000.0
Net realized and unrealized investment gains													318,500,000
Net realized and unrealized investment gains (losses)													433,200,000	(108,300,000)	133,300,000
Commission revenues													249,100,000	389,200,000	167,000,000.0
Other revenue													47,600,000	15,300,000	7,100,000
Total revenues		$ 175,300,000	$ 155,500,000	$ 129,000,000.0	$ 433,600,000	$ 6,000,000.0	$ 198,700,000	$ 122,300,000	$ 42,100,000				893,400,000	369,100,000	373,800,000
Expenses
Loss and loss adjustment expenses															1,100,000
Insurance acquisition expense													5,700,000	5,300,000	4,100,000
Other underwriting expenses													400,000	400,000	400,000
Cost of sales													48,100,000	248,700,000	139,400,000
General and administrative expenses													327,800,000	233,500,000	208,000,000.0
Broker commission expense													64,800,000	28,400,000
Change in fair value of contingent consideration earnout liabilities													2,100,000	2,700,000
Amortization of other intangible assets													21,800,000	18,800,000	10,700,000
Interest expense on debt													17,600,000	9,500,000	2,300,000
Total expenses		118,800,000	104,400,000	113,200,000	151,900,000	150,100,000	154,400,000	134,700,000	108,100,000				488,300,000	547,300,000	366,000,000.0
Pre-tax income (loss)		56,500,000	51,100,000	15,800,000	281,700,000	(144,100,000)	44,300,000	(12,400,000)	(66,000,000.0)				405,100,000	(178,200,000)	7,800,000
Income tax (expense) benefit		(10,400,000)	(8,800,000)	100,000	(10,200,000)	3,600,000	3,600,000	(2,500,000)	(700,000)				(29,300,000)	4,000,000.0	7,800,000
Net income (loss) from continuing operations		46,100,000	42,300,000	15,900,000	271,500,000	(140,500,000)	47,900,000	(14,900,000)	(66,700,000)				375,800,000	(174,200,000)	15,600,000
Gain (loss) on sale of other discontinued operations, net of tax													800,000	(17,200,000)	557,000,000.0
Net income from discontinued operations, net of tax		(800,000)	900,000	0	700,000	0	(17,300,000)	0	100,000				0	0	20,500,000
Net income													376,600,000	(191,400,000)	593,100,000
Net loss attributable to non-controlling interests		15,600,000	5,500,000	4,600,000	12,200,000	3,000,000.0	10,200,000	18,400,000	18,600,000				37,900,000	50,200,000	34,100,000
Net income attributable to White Mountains’s common shareholders		$ 60,900,000	$ 48,700,000	$ 20,500,000	$ 284,400,000	$ (137,500,000)	$ 40,800,000	$ 3,500,000	$ (48,000,000.0)				414,500,000	(141,200,000)	627,200,000
Kudu
Revenues
Earned insurance premiums													0
Net investment income													14,700,000	0	0
Net realized and unrealized investment gains													6,300,000
Net realized and unrealized investment gains (losses)														0	0
Commission revenues													0
Other revenue													200,000	0	0
Total revenues													21,200,000	0	0
Expenses
Insurance acquisition expense													0
Other underwriting expenses													0
Cost of sales													0
General and administrative expenses													10,100,000	0	0
Broker commission expense													0
Amortization of other intangible assets													200,000	0	0
Interest expense on debt													100,000	0	0
Total expenses													10,400,000	0	0
Pre-tax income (loss)													10,800,000
HG Global-BAM
Revenues
Net investment income													21,600,000	16,700,000	12,300,000
Net realized and unrealized investment gains													27,100,000
Net realized and unrealized investment gains (losses)														(7,500,000)	600,000
Commission revenues													0
Other revenue													1,600,000	1,200,000	1,000,000.0
Total revenues													66,600,000	24,300,000	23,300,000
Expenses
Insurance acquisition expense													5,700,000	5,300,000	4,000,000.0
Other underwriting expenses													400,000	400,000	400,000
Cost of sales													0
General and administrative expenses													50,500,000	48,000,000.0	42,900,000
Broker commission expense													0
Amortization of other intangible assets													0
Interest expense on debt													0
Total expenses													56,600,000	53,700,000	47,300,000
Pre-tax income (loss)													10,000,000.0	(29,400,000)	(24,000,000.0)
NSM Holding Co LLC
Revenues
Earned insurance premiums													0
Net investment income													0
Net realized and unrealized investment gains													0
Commission revenues													193,400,000	89,600,000	0
Other revenue													39,700,000	12,000,000.0	0
Total revenues													233,100,000	101,600,000	0
Expenses
Insurance acquisition expense													0
Other underwriting expenses													0
Cost of sales													0
General and administrative expenses													132,200,000	59,400,000	0
Broker commission expense													64,800,000	28,400,000	0
Change in fair value of contingent consideration earnout liabilities													2,100,000	2,700,000	0
Amortization of other intangible assets													19,400,000	8,300,000	0
Interest expense on debt													16,700,000	8,000,000.0	0
Total expenses													235,200,000	106,800,000	0
Pre-tax income (loss)													(2,100,000)	(5,200,000)
MediaAlpha
Revenues
Commission revenues													48,800,000	295,500,000	163,200,000
Other revenue													0	1,600,000	0
Total revenues													48,800,000	297,100,000	163,200,000
Expenses
Cost of sales													40,600,000	245,000,000.0	135,900,000
General and administrative expenses													12,500,000	31,700,000	16,200,000
Amortization of other intangible assets													1,600,000	10,300,000	10,500,000
Interest expense on debt													200,000	1,200,000	1,000,000.0
Total expenses													54,900,000	288,200,000	163,600,000
Pre-tax income (loss)													(6,100,000)	8,900,000	(400,000)
Other
Revenues
Earned insurance premiums													0	0	1,000,000.0
Net investment income													43,400,000	42,300,000	43,700,000
Net realized and unrealized investment gains													285,100,000
Net realized and unrealized investment gains (losses)														(100,800,000)	132,700,000
Realized gain and unrealized investment gain from the MediaAlpha Transaction														0	0
Commission revenues													6,900,000	4,100,000	3,800,000
Other revenue													6,100,000	500,000	6,100,000
Total revenues													523,700,000	(53,900,000)	187,300,000
Expenses
Loss and loss adjustment expenses													0	0	1,100,000
Insurance acquisition expense													0	0	100,000
Other underwriting expenses													0
Cost of sales													7,500,000	3,700,000	3,500,000
General and administrative expenses													122,500,000	94,400,000	148,900,000
Broker commission expense													0
Amortization of other intangible assets													600,000	200,000	200,000
Interest expense on debt													600,000	300,000	1,300,000
Total expenses													131,200,000	98,600,000	155,100,000
Pre-tax income (loss)													392,500,000	(152,500,000)	32,200,000
NSM Holding Co LLC
Expenses
Change in fair value of contingent consideration earnout liabilities										$ 2,700,000			2,100,000
MediaAlpha | MediaAlpha
Revenues
Total revenues												$ 359,200,000
MediaAlpha | Other
Revenues
Realized gain and unrealized investment gain from the MediaAlpha Transaction													182,200,000
OneBeacon
Expenses
Gain (loss) on sale of other discontinued operations, net of tax	$ 554,600,000
Net income from discontinued operations, net of tax											$ 20,500,000
Tranzact
Expenses
Gain (loss) on sale of other discontinued operations, net of tax														3,200,000	3,200,000
Sirius Group
Expenses
Gain (loss) on sale of other discontinued operations, net of tax														(17,300,000)
Discontinued Operations
Expenses
Net income attributable to White Mountains’s common shareholders													414,500,000	(141,200,000)	627,200,000
Discontinued Operations | OneBeacon
Revenues
Earned insurance premiums															807,600,000
Net investment income															39,700,000
Net realized and unrealized investment gains (losses)															38,800,000
Other revenue															7,700,000
Total revenues															893,800,000
Expenses
Loss and loss adjustment expenses															546,000,000.0
Other underwriting expenses															156,200,000
General and administrative expenses															21,200,000
Interest expense on debt															10,000,000.0
Total expenses															879,000,000.0
Gain (loss) on sale of other discontinued operations, net of tax															554,500,000
Net income from discontinued operations, net of tax															20,500,000
Discontinued Operations | Tranzact
Revenues
Earned insurance premiums															0
Net investment income															0
Net realized and unrealized investment gains (losses)															0
Other revenue															0
Total revenues															0
Expenses
Loss and loss adjustment expenses															0
Other underwriting expenses															0
General and administrative expenses															0
Interest expense on debt															0
Total expenses															0
Gain (loss) on sale of other discontinued operations, net of tax															3,200,000
Net income from discontinued operations, net of tax															0
Discontinued Operations | Sirius Group
Revenues
Earned insurance premiums															0
Net investment income															0
Net realized and unrealized investment gains (losses)															0
Other revenue															0
Total revenues															0
Expenses
Loss and loss adjustment expenses															0
Other underwriting expenses															0
General and administrative expenses															0
Interest expense on debt															0
Total expenses															0
Gain (loss) on sale of other discontinued operations, net of tax													$ 800,000	$ (17,200,000)	(700,000)
Net income from discontinued operations, net of tax															$ 0

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