Derivatives (Pre-Tax Operating Results of WM Life Re) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Pre-tax operating results of WM Life Re
Total revenues	$ 626.6	$ 574.1	$ 489.3	$ 627.4	$ 566.4	$ 697.4	$ 542.3	$ 629.7	$ 2,317.4		$ 2,435.7		$ 2,173.1
General and administrative expenses									(181.3)		(182.2)		(175.3)
Pre-tax income	134.2	55.3	3.6	151.8	(50.8)	135.1	37.6	140.9	344.9		262.8		98.0
White Mountains Life Re
Pre-tax operating results of WM Life Re
Fees, included in other revenue									25.0		31.8		32.5
Change in Fair Value of Variable Annuity Liability, included in other revenues									378.5		312.8		(156.5)
Change in fair value of derivatives, included in other revenue									(402.0)		(339.0)		92.9
Foreign exchange, included in other revenue									(14.5)		(30.3)		15.1
Other investment income and (losses) gains									(7.1)		2.5		(0.9)
Total revenues									(20.1)		(22.2)		(16.9)
Change in fair value of variable annuity death benefit liabilities, included in general and administrative expenses									10.2		14.2		(1.8)
Death benefit claims paid, included in general and administrative expenses									(1.9)		(5.7)		(3.8)
General and administrative expenses									(4.9)		(5.2)		(4.7)
Pre-tax income									(16.7)		(18.9)		(27.2)
Level 3 | White Mountains Life Re
Pre-tax operating results of WM Life Re
Change in fair value of derivatives, included in other revenue									136.5	[1]	84.0	[1]	(14.5)	[1]
Level 3 | Projected Surrender Assumption [Member] | White Mountains Life Re
Pre-tax operating results of WM Life Re
Change in fair value of derivatives, included in other revenue									$ 6.5				$ 7.2

[1] Consists of over-the-counter instruments.