Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments (Details) (White Mountains Life Re [Member], USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011		Dec. 31, 2011
Changes in variable annuity liabilities
Derivative, Fair Value, Net					$ 290.4
Derivative, Gain (Loss) on Derivative, Net	(11.0)		148.9		(122.0)		105.4
Derivative, Fair Value, Net	242.1				242.1
Collateral Cash and Investments	477.5		419.7		477.5		419.7		485.3
Fair Value
Changes in variable annuity liabilities
Derivative, Fair Value, Net	250.3		269.0	[1]	290.4		347.5	[1]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0		0	[1]	6.1		5.0	[1]
Derivative, Gain (Loss) on Derivative, Net	(11.0)		148.9	[1]	(122.0)		105.4	[1]
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements	2.8		(64.1)	[1]	67.6		(104.1)	[1]
Derivative, Fair Value, Net	242.1		353.8	[1]	242.1		353.8	[1]
Level 3 Inputs
Changes in derivative instruments
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	(661.8)		(591.3)		(768.5)		(610.2)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases	0		0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings	(10.6)		(170.0)		96.1		(151.1)
Fair Value Measurement with Unobservable Inputs, Reconciliation, Recurring Basis, Liability Sales or Settlements	0		0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	(672.4)		(761.3)		(672.4)		(761.3)
Changes in variable annuity liabilities
Derivative, Fair Value, Net	213.0	[2]	232.7	[2]	247.1	[2]	275.3	[2]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0	[2]	0	[2]	6.1	[2]	5.0	[2]
Derivative, Gain (Loss) on Derivative, Net	6.7	[2]	45.6	[2]	(24.1)	[2]	27.1	[2]
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements	(6.7)	[2]	0	[2]	(16.1)	[2]	(29.1)	[2]
Derivative, Fair Value, Net	213.0	[2]	278.3	[2]	213.0	[2]	278.3	[2]
Level 2 Inputs
Changes in variable annuity liabilities
Derivative, Fair Value, Net	61.0	[2]	48.2	[2],[3]	39.2	[2]	72.2	[2],[3]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0	[2]	0	[2],[3]	0	[2]	0	[2],[3]
Derivative, Gain (Loss) on Derivative, Net	(15.3)	[2]	75.7	[2],[3]	(71.8)	[2]	67.5	[2],[3]
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements	(12.5)	[2]	(48.4)	[2],[3]	65.8	[2]	(64.2)	[2],[3]
Derivative, Fair Value, Net	33.2	[2]	75.5	[2],[3]	33.2	[2]	75.5	[2],[3]
Level 1 Inputs
Changes in variable annuity liabilities
Derivative, Fair Value, Net	(23.7)		(11.9)	[4]	4.1		0	[4]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0		0	[4]	0		0	[4]
Derivative, Gain (Loss) on Derivative, Net	(2.4)		27.6	[4]	(26.1)		10.8	[4]
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements	22.0		(15.7)	[4]	17.9		(10.8)	[4]
Derivative, Fair Value, Net	$ (4.1)		$ 0	[4]	$ (4.1)		$ 0	[4]

[1] (4) In addition to derivative instruments, WM Life Re held cash, short-term and fixed maturity investments of $477.5 and $419.7 at September 30, 2012 and 2011 posted as collateral to its reinsurance counterparties.

[2] (1) Consists of over-the-counter instruments.

[3] (2) 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] (3) 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.