CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY (USD $) In Millions, unless otherwise specified	Total	White Mountains' Common Shareholders' Equity	Common shares and paid-in surplus	Retained earnings	Accum. other comprehensive income (loss), after-tax	Noncontrolling interest
Balances at Dec. 31, 2008		$ 2,898.8	$ 1,428.2	$ 1,751.9	$ (281.3)	$ 613.7
Increase (Decrease) in Shareholders' Equity
Cumulative effect adjustment (Symetra FAS115-2)				2.9	(2.9)
Net income	579.5	470.0		470.0		109.5
Net change in unrealized gains (losses) from investments in unconsolidated affiliates	192.4	192.4			192.4
Net change in foreign currency translation	73.0	73.0			73.0
Net change in pension liability and other accumulated comprehensive items	19.1	15.3			15.3	3.7
Dividends declared on common shares	(8.9)	(8.9)		(8.9)
Dividends/distributions to noncontrolling interests						(38.4)
Issuances of common shares		0.9	0.9
Repurchase and retirement of common shares		(0.4)	(0.4)
Distributions to noncontrolling interests in limited partnerships						(5.0)
Contributions from noncontrolling interests in OneBeacon						0.3
Amortization of restricted share and option awards		16.3	16.3			0.3
Balances at Dec. 31, 2009		3,657.4	1,445.0	2,215.9	(3.5)	684.1
Increase (Decrease) in Shareholders' Equity
Cumulative effect adjustment (ASU 2009-17)		(0.4)		(0.4)		(22.8)
Tax basis change due to intercompany transfer		(10.4)		(4.4)	(6.0)
Net income	139.5	86.5		86.5		53.0
Net change in unrealized gains (losses) from investments in unconsolidated affiliates	73.5	73.5			73.5
Net change in foreign currency translation	49.8	49.8			49.8
Net change in pension liability and other accumulated comprehensive items	6.3	4.6			4.6	1.7
Dividends declared on common shares	(8.8)	(8.8)		(8.8)
Dividends/distributions to noncontrolling interests						(94.7)
Issuances of common shares		0.7	0.7
Repurchase and retirement of common shares	(222.0)	(225.6)	(112.4)	(113.2)
Distributions to noncontrolling interests in limited partnerships						(13.6)
Amortization of restricted share and option awards		25.7	25.7			0.1
Balances at Dec. 31, 2010	4,260.8	3,653.0	1,359.0	2,175.6	118.4	607.8
Increase (Decrease) in Shareholders' Equity
Net income	809.4	767.9		767.9		41.5
Net change in unrealized gains (losses) from investments in unconsolidated affiliates	(58.5)	(58.5)			(58.5)
Net change in foreign currency translation	(15.2)	(15.2)			(15.2)
Net change in pension liability and other accumulated comprehensive items	(10.8)	(8.0)			(8.0)	(2.8)
Dividends declared on common shares		(8.0)		(8.0)
Dividends/distributions to noncontrolling interests						(61.5)
Issuances of common shares		0.9	0.9
Repurchase and retirement of common shares	(114.2)	(253.0)	(107.2)	(145.8)
Distributions to noncontrolling interests in limited partnerships						(8.6)
Amortization of restricted share and option awards		12.0	12.0			0.4
Noncontrolling interest attributable to intercompany sale of subsidiary		(3.4)	(3.4)			3.4
Balances at Dec. 31, 2011	$ 4,667.9	$ 4,087.7	$ 1,261.3	$ 2,789.7	$ 36.7	$ 580.2