Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) - White Mountains Life Re $ in Thousands	Dec. 31, 2015 USD ($) Rating	Dec. 31, 2014 USD ($)
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	$ 20,100	$ 56,400
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	$ 19,000	77,200
S&P Credit Ratings | Rating	21
Derivative, Collateral, Right to Reclaim Cash	$ 0	3,500
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	20,100	59,900
Excess collateral provided to counterparty in cash	5,800	17,700
Excess collateral providedto counterparty in financial instruments	0	9,500
Derivative, Collateral, Obligation to Return Cash	6,900	9,900
A | Citigroup OTC [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	9,900	22,200
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	8,500	21,100
Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	9,900	22,200
Excess collateral provided to counterparty in cash	0	0
Excess collateral providedto counterparty in financial instruments	0	0
Derivative, Collateral, Obligation to Return Cash	1,400	1,100
A | Royal Bank of Scotland [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets		4,000
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments		4,000
Derivative, Collateral, Right to Reclaim Cash		0
Derivative, Collateral, Right to Reclaim Securities		0
Derivative Asset, Fair Value, Amount Offset Against Collateral		4,000
Excess collateral provided to counterparty in cash		0
Excess collateral providedto counterparty in financial instruments		0
Derivative, Collateral, Obligation to Return Cash		0
A | Barclays [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets		100
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments		100
Derivative, Collateral, Right to Reclaim Cash		0
Derivative, Collateral, Right to Reclaim Securities		0
Derivative Asset, Fair Value, Amount Offset Against Collateral		100
Excess collateral provided to counterparty in cash		0
Excess collateral providedto counterparty in financial instruments		0
Derivative, Collateral, Obligation to Return Cash		0
A | Bank of America [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	700	5,600
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	700	5,600
Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	700	5,600
Excess collateral provided to counterparty in cash	0	0
Excess collateral providedto counterparty in financial instruments	0	0
Derivative, Collateral, Obligation to Return Cash	0	0
A | Citigroup Exchange Traded [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	1,000	3,700
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	6,800	19,700
Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	1,000	3,700
Excess collateral provided to counterparty in cash	5,800	16,000
Excess collateral providedto counterparty in financial instruments	0	0
Derivative, Collateral, Obligation to Return Cash	0	0
BBB plus | Nomura [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets		(3,500)
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments		11,200
Derivative, Collateral, Right to Reclaim Cash		3,500
Derivative, Collateral, Right to Reclaim Securities		0
Derivative Asset, Fair Value, Amount Offset Against Collateral		0
Excess collateral provided to counterparty in cash		1,700
Excess collateral providedto counterparty in financial instruments		9,500
Derivative, Collateral, Obligation to Return Cash		0
Standard & Poor's, A+ Rating [Member] | J P Morgan Chase [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	8,500	24,300
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	3,000	15,500
Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	8,500	24,300
Excess collateral provided to counterparty in cash	0	0
Excess collateral providedto counterparty in financial instruments	0	0
Derivative, Collateral, Obligation to Return Cash	$ 5,500	$ 8,800