Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) - White Mountains Life Re $ in Thousands	Sep. 30, 2015 USD ($) Rating	Dec. 31, 2014 USD ($)
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	$ 37,000	$ 56,400
Collertal provided to counterparty in cash	2,200	3,500
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	39,200	59,900
Excess collateral provided to counterparty in cash	14,200	17,700
Derivative, Collateral, Right to Reclaim Securities	0	9,500
Counter-party collateral held by WMLife Re - Cash	11,200	9,900
Net amount of exposure to counter-party	$ 42,200	77,200
S and P Credit Ratings | Rating	23
Standard & Poor's, A Rating | Bank of America [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	$ 3,200	5,600
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	3,200	5,600
Excess collateral provided to counterparty in cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	0	0
Net amount of exposure to counter-party	3,200	5,600
Standard & Poor's, A Rating | Barclays
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	0	100
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	0	100
Excess collateral provided to counterparty in cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	0	0
Net amount of exposure to counter-party	0	100
Standard & Poor's, A Rating | Royal Bank of Scotland
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments		4,000
Collertal provided to counterparty in cash		0
Collateral provided to counterparty in financial instruments		0
Net amount of exposure after effect of collateral		4,000
Excess collateral provided to counterparty in cash		0
Derivative, Collateral, Right to Reclaim Securities		0
Counter-party collateral held by WMLife Re - Cash		0
Net amount of exposure to counter-party		4,000
Standard & Poor's, A Rating | Citigroup - OTC
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	18,600	22,200
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	18,600	22,200
Excess collateral provided to counterparty in cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	5,300	1,100
Net amount of exposure to counter-party	13,300	21,100
Standard & Poor's, A Rating | Citigroup Exchange Traded [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	(2,100)	3,700
Collertal provided to counterparty in cash	2,100	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	0	3,700
Excess collateral provided to counterparty in cash	12,800	16,000
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	0	0
Net amount of exposure to counter-party	12,800	19,700
Standard & Poor's, A plus | JP Morgan
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	15,900	24,300
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	15,900	24,300
Excess collateral provided to counterparty in cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	5,900	8,800
Net amount of exposure to counter-party	10,000	15,500
Standard & Poor's, BBB plus Rating | Royal Bank of Scotland
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	1,500
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	1,500
Excess collateral provided to counterparty in cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Counter-party collateral held by WMLife Re - Cash	0
Net amount of exposure to counter-party	1,500
Standard & Poor's, BBB plus Rating | Nomura [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	(100)	(3,500)
Collertal provided to counterparty in cash	100	3,500
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	0	0
Excess collateral provided to counterparty in cash	1,400	1,700
Derivative, Collateral, Right to Reclaim Securities	0	9,500
Counter-party collateral held by WMLife Re - Cash	0	0
Net amount of exposure to counter-party	$ 1,400	$ 11,200