Derivatives (Pre-Tax Operating Results of WM Life Re) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended					9 Months Ended
	Sep. 30, 2013		Jun. 30, 2013	Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Pre-tax operating results of WM Life Re
Total revenues	$ 574.1			$ 697.4		$ 1,690.8		$ 1,869.3
General and administrative expenses	(41.5)			(58.7)		(125.5)		(146.3)
Pre-tax income from continuing operations	55.3			135.1		210.7		313.6
White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Realized and unrealized gains (losses)			1.5
Pre-tax operating results of WM Life Re
Fees, included in other revenue	6.2			8.1		19.2		24.1
Change in fair value of variable annuity liability, included in other revenue	49.1			(11.3)		289.3		89.4
Realized and unrealized (losses) gains	(63.1)			(11.0)		(308.4)		(122.0)
Foreign exchange, included in other revenue	0.6			9.0		(13.9)		(6.4)
Other investment income and gains (losses)	0.2			3.8		(5.2)		2.9
Total revenues	(7.0)			(1.4)		(19.0)		(12.0)
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	1.3			0.7		8.5		6.7
Death benefit claims paid, included in general and administrative expenses	(0.2)			(1.5)		(1.6)		(4.9)
General and administrative expenses	(1.0)			(1.1)		(3.8)		(3.8)
Pre-tax income from continuing operations	(6.9)			(3.3)		(15.9)		(14.0)
Fair Value, Inputs, Level 3 [Member] | White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Realized and unrealized gains (losses)	50.4	[1]		(10.6)	[1]	297.8	[1]	96.1	[1]
Pre-tax operating results of WM Life Re
Realized and unrealized (losses) gains	$ 24.1	[2]		$ (6.7)	[2]	$ 97.4	[2]	$ 24.1	[2]

[1] Includes $(1.3) and $(8.5) for the three and nine months ended September 30, 2013 and $(0.7) and $(6.7) for the three and nine months ended September 30, 2012 related to the change in the fair value of variable annuity death benefit liabilities, which are included in general and administrative expenses.

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