Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012		Dec. 31, 2012
Variable Annuity Reinsurance [Line Items]
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	$ 5.6		$ 8.8
Changes in derivative instruments
Beginning balance	98.3
Realized and unrealized gains (losses)	181.8		206.5
Ending balance	75.8
Collateral cash and investments	229.8		390.9		393.6
Fair Value
Changes in derivative instruments
Beginning balance	98.3		290.4
Purchases	36.1		6.1
Realized and unrealized gains (losses)	(181.8)		(206.5)
Transfers in	0		0
Sales/settlements	123.2		84.3
Ending balance	75.8		174.3
Level 3 Inputs
Changes in variable annuity liabilities
Beginning balance	(441.5)		(768.5)
Purchases	0		0
Realized and unrealized gains (losses)	189.9	[1]	220.0	[1]
Transfers in	0		0
Sales/settlements			0
Ending balance	(251.6)		(548.5)
Changes in derivative instruments
Beginning balance	140.5	[2]	247.1	[2]
Purchases	36.1	[2]	6.1	[2]
Realized and unrealized gains (losses)	(49.2)	[2]	(60.3)	[2]
Transfers in	0	[2]	0	[2]
Sales/settlements		[2]	(9.1)	[2]
Ending balance	127.4	[2]	183.8	[2]
Level 2 Inputs
Changes in derivative instruments
Beginning balance	(20.5)	[2],[3]	39.2	[2],[3]
Purchases	0	[2],[3]	0	[2],[3]
Realized and unrealized gains (losses)	(85.5)	[2],[3]	(98.2)	[2],[3]
Transfers in	0	[2],[3]	0	[2],[3]
Sales/settlements	63.1	[2],[3]	58.9	[2],[3]
Ending balance	(42.9)	[2],[3]	(0.1)	[2],[3]
Level 1 Inputs
Changes in derivative instruments
Beginning balance	(21.7)	[4]	4.1	[4]
Purchases	0	[4]	0	[4]
Realized and unrealized gains (losses)	(47.1)	[4]	(48.0)	[4]
Transfers in	0	[4]	0	[4]
Sales/settlements	60.1	[4]	34.5	[4]
Ending balance	$ (8.7)	[4]	$ (9.4)	[4]

[1] Includes $5.6 and $8.8 for the three months ended March 31, 2013 and 2012, respectively, related to the increase in the fair value of variable annuity death benefit liabilities, which are included in other general and administrative expenses.

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

[3] Consists of interest rate swaps, total return swaps, foreign currency forward contracts, and bond forwards. Fair value measurement based upon bid/ask pricing quotes for similar instruments that are actively traded, where available. Swaps for which an active market does not exist have been priced using observable inputs including the swap curve and the underlying bond index.

[4] Consists of exchange traded equity index, foreign currency and interest rate futures. Fair value measurements based upon quoted prices for identical instruments that are actively traded.