Derivatives (Pre-Tax Operating Results of WM Life Re) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pre-tax operating results of WM Life Re
Revenues	$ 697.4	$ 501.8	$ 1,869.3	$ 1,579.7
General and administrative expenses	(58.7)	(36.2)	(146.3)	(120.4)
Pre-tax income (loss) from continuing operations	135.1	1.3	313.6	(0.4)
White Mountains Life Re [Member]
Pre-tax operating results of WM Life Re
Policy Charges, Insurance	8.1	8.4	24.1	24.3
Gain (Loss) on Variable Annuity Liabilities	(11.3)	(164.4)	89.4	(148.0)
Derivative, Gain (Loss) on Derivative, Net	(11.0)	148.9	(122.0)	105.4
Foreign Exchange Derivatives Gain (Loss)	9.0	14.8	(6.4)	17.1
Other Investment Income and Gain (Loss) on Investments	3.8	0.5	2.9	(0.1)
Revenues	(1.4)	8.2	(12.0)	(1.3)
Gain (Loss) on Liability for Death Benefit	0.7	(5.6)	6.7	(3.1)
Liability for Future Policy Benefits, Benefits Paid	(1.5)	(0.7)	(4.9)	(2.4)
General and administrative expenses	(1.1)	(1.1)	(3.8)	(3.2)
Pre-tax income (loss) from continuing operations	$ (3.3)	$ 0.8	$ (14.0)	$ (10.0)