Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) - White Mountains Life Re $ in Thousands	Jun. 30, 2015 USD ($) Rating	Dec. 31, 2014 USD ($)
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	$ 35,800	$ 56,400
Collertal provided to counterparty in cash	5,300	3,500
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	41,100	59,900
Excess collateral provided to counterparty in cash	15,600	17,700
Derivative, Collateral, Right to Reclaim Securities	4,400	9,500
Counter-party collateral held by WMLife Re - Cash	11,400	9,900
Net amount of exposure to counter-party	$ 49,700	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	$ 2,000	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	2,000	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	2,000	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	21,300	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	21,300	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	4,900	1,100
Net amount of exposure to counter-party	16,400	21,100
Standard & Poor's, A Rating | Citigroup Exchange Traded [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	(5,000)	3,700
Collertal provided to counterparty in cash	5,000	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	10,700	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	10,700	19,700
Standard & Poor's, A plus | JP Morgan
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	16,800	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	16,800	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	6,500	8,800
Net amount of exposure to counter-party	10,300	15,500
Standard & Poor's, BBB plus Rating | Nomura [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	(300)	(3,500)
Collertal provided to counterparty in cash	300	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	4,900	1,700
Derivative, Collateral, Right to Reclaim Securities	4,400	9,500
Counter-party collateral held by WMLife Re - Cash	0	0
Net amount of exposure to counter-party	9,300	$ 11,200
Standard & Poor's, A- Rating | Royal Bank of Scotland
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	1,000
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	1,000
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	$ 1,000