SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details 2) (USD $)	3 Months Ended								12 Months Ended			1 Months Ended	12 Months Ended						1 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Nov. 30, 2011 Lone Tree Insurance Group Ltd [Member]	Dec. 31, 2012 White Mountains Insurance Group Ltd.		Dec. 31, 2011 White Mountains Insurance Group Ltd.		Dec. 31, 2010 White Mountains Insurance Group Ltd.		Nov. 30, 2011 Fixed maturity investments Lone Tree Insurance Group Ltd [Member]	Nov. 30, 2011 Short-term investments Lone Tree Insurance Group Ltd [Member]
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Payments to subsidiaries													$ (663,000,000)	[1]	$ 0	[1]	$ 0	[1]
Noncash investments acquired																			0	284.7
Noncash payables acquired												417.5
Revenues (including realized gains and losses)	566,400,000	697,400,000	542,300,000	629,700,000	593,400,000	501,700,000	552,400,000	525,600,000	2,435,700,000	2,173,100,000	2,333,700,000		20,800,000		6,400,000		1,100,000
Expenses	617,200,000	562,300,000	504,700,000	488,800,000	495,000,000	500,500,000	516,600,000	563,000,000	2,172,900,000	2,075,100,000	2,144,400,000		32,400,000		51,700,000		42,200,000
Pre-tax income (loss)	(50,800,000)	135,100,000	37,600,000	140,900,000	98,400,000	1,200,000	35,800,000	(37,400,000)	262,800,000	98,000,000	189,300,000		(11,600,000)		(45,300,000)		(41,100,000)
Income tax benefit (expense)	101,000,000	(47,800,000)	(6,400,000)	(31,100,000)	110,900,000	600,000	(8,800,000)	7,300,000	15,700,000	110,000,000	(29,600,000)		(300,000)		6,500,000		(600,000)
Net income from continuing operations	50,200,000	87,300,000	31,200,000	109,800,000	209,300,000	1,800,000	27,000,000	(30,100,000)	278,500,000	208,000,000	159,700,000		(11,900,000)		(38,800,000)		(41,700,000)
Equity in earnings (losses) from consolidated and unconsolidated affiliates													219,300,000	[2]	806,700,000	[2]	128,200,000	[2]
Net income attributable to White Mountains’ common shareholders	68,200,000	19,100,000	26,100,000	94,000,000	800,200,000	(15,800,000)	11,700,000	(28,200,000)	207,400,000	767,900,000	86,500,000		207,400,000	[3]	767,900,000	[3]	86,500,000	[3]
Other comprehensive income (loss), after-tax													95,200,000		(81,700,000)		127,900,000
Comprehensive income attributable to White Mountains’ common shareholders									302,600,000	686,200,000	214,400,000		302,600,000		686,200,000		214,400,000
Computation of net income (loss) available to common shareholders:
Net Income (Loss) Available to Common Stockholders, Basic	68,200,000	19,100,000	26,100,000	94,000,000	800,200,000	(15,800,000)	11,700,000	(28,200,000)	207,400,000	767,900,000	86,500,000		207,400,000	[3]	767,900,000	[3]	86,500,000	[3]
Common shares repurchased and retired during the period									1,329,640	646,502	687,871
Payments for Repurchase of Common Stock									$ 669,100,000	$ 253,000,000	$ 225,600,000		$ 669,100,000	[1]	$ 253,000,000	[1]	$ 225,600,000	[1]

[1] During 2012, the Company sold the majority of its fixed maturity investments and used the proceeds to (a) contribute $663.0 to its subsidiaries, the majority of which was used to fund HG Global and (b) repurchase 1,329,640 of its common shares for $669.1

[2] (3) In 2011, the Company revised the presentation of investments in consolidated and unconsolidated affiliates, total liabilities and equity and equity in earnings (losses) from consolidated and unconsolidated affiliates to be net of non-controlling interests for all periods presented.

[3] n 2011, the Company revised the presentation of net income attributable to White Mountains’ common shareholders and undistributed current earnings from subsidiaries to be net of non-controlling interests for all periods presented.