CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (USD $) In Millions, unless otherwise specified	Total	Common shares and paid-in surplus White Mountains	Retained earnings White Mountains	Accum. Other comprehensive income (loss),after-tax White Mountains	White Mountains excluding noncontrolling interest White Mountains	Noncontrolling interest	Total Equity includes Noncontrolling Interest
Balances at Dec. 31, 2011		$ 1,261.3	$ 2,789.7	$ 36.7	$ 4,087.7	$ 580.2	$ 4,667.9
Increase (Decrease) in Shareholders' Equity
Net income	148.8	0	119.9	0	119.9	28.9	148.8
Net change in unrealized losses from investments in unconsolidated affiliates	27.0	0	0	27.0	27.0	0	27.0
Net change in foreign currency translation	(6.3)	0	0	(6.3)	(6.3)	0	(6.3)
Net change in pension liability and other accumulated comprehensive items		0	0	0	0	0	0
Total comprehensive income	140.6	0	119.9	20.7	140.6	28.9	169.5
Dividends declared on common shares		0	(6.6)	0	(6.6)	0	(6.6)
Dividends to noncontrolling interests		0	0	0	0	(19.3)	(19.3)
Repurchases and retirements of common shares	(491.0)	(164.3)	(326.7)	0	(491.0)	0	(491.0)
Issuances of common shares		5.5	0	0	5.5	0	5.5
Net distributions to noncontrolling interests		0	0	0	0	1.8	1.8
Amortization of restricted share and option awards		6.3	0	0	6.3	0.4	6.7
Balances at Jun. 30, 2012		1,108.8	2,576.3	57.4	3,742.5	592.0	4,334.5
Balances at Dec. 31, 2012	4,258.2	1,057.2	2,542.7	131.9	3,731.8	526.4	4,258.2
Increase (Decrease) in Shareholders' Equity
Net income	135.1	0	146.7	0	146.7	(11.6)	135.1
Net change in unrealized losses from investments in unconsolidated affiliates	(74.0)	0	0	(74.0)	(74.0)	0	(74.0)
Net change in foreign currency translation	(38.3)	0	0	(38.5)	(38.5)	0	(38.5)
Net change in pension liability and other accumulated comprehensive items		0	0	0.2	0.2	0	0.2
Total comprehensive income	34.4	0	146.7	(112.3)	34.4	(11.6)	22.8
Dividends declared on common shares		0	(6.2)	0	(6.2)	0	(6.2)
Dividends to noncontrolling interests		0	0	0	0	(19.8)	(19.8)
Repurchases and retirements of common shares	(79.8)	(23.8)	(56.0)	0	(79.8)	0	(79.8)
Issuances of common shares		1.0	0	0	1.0	0	1.0
Net distributions to noncontrolling interests		0	0	0	0	5.5	5.5
Amortization of restricted share and option awards		8.0	0	0	8.0	0.7	8.7
Balances at Jun. 30, 2013	$ 4,190.4	$ 1,042.4	$ 2,627.2	$ 19.6	$ 3,689.2	$ 501.2	$ 4,190.4