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	Non-controlling 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	110.6	0	93.8	0	93.8	16.8	110.6
Net change in unrealized losses from investments in unconsolidated affiliates	(1.2)	0	0	(1.2)	(1.2)	0	(1.2)
Change in foreign currency translation and other	23.5	0	0	23.5	23.5	0	23.5
Net change in pension liability and other accumulated comprehensive items		0	0	0	0	0	0
Total comprehensive income	116.1	0	93.8	22.3	116.1	16.8	132.9
Dividends declared on common shares		0	(6.6)	0	(6.6)	0	(6.6)
Dividends to noncontrolling interests		0	0	0	0	(5.0)	(5.0)
Repurchases and retirements of common shares	(485.8)	(162.6)	(323.2)	0	(485.8)	0	(485.8)
Issuances of common shares		4.6	0	0	4.6	0	4.6
Net distributions to noncontrolling interests		0	0	0	0	(1.5)	(1.5)
Amortization of restricted share and option awards		2.8	0	0	2.8	0.2	3.0
Balances at Mar. 31, 2012		1,106.1	2,553.7	59.0	3,718.8	590.7	4,309.5
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	119.9	0	120.4	0	120.4	(0.5)	119.9
Net change in unrealized losses from investments in unconsolidated affiliates	(8.0)	0	0	(8.0)	(8.0)	0	(8.0)
Change in foreign currency translation and other	(3.7)	0	0	(3.7)	(3.7)	0	(3.7)
Net change in pension liability and other accumulated comprehensive items		0	0	0	0	0	0
Total comprehensive income	108.7	0	120.4	(11.7)	108.7	(0.5)	108.2
Dividends declared on common shares		0	(6.2)	0	(6.2)	0	(6.2)
Dividends to noncontrolling interests		0	0	0	0	(5.2)	(5.2)
Repurchases and retirements of common shares	(79.0)	(23.5)	(55.5)	0	(79.0)	0	(79.0)
Issuances of common shares		0	0	0	0	0	0
Net distributions to noncontrolling interests		0	0	0	0	2.2	2.2
Amortization of restricted share and option awards		3.6	0	0	3.6	0.2	3.8
Balances at Mar. 31, 2013	$ 4,282.0	$ 1,037.3	$ 2,601.4	$ 120.2	$ 3,758.9	$ 523.1	$ 4,282.0