Variable Annuity Reinsurance and Weather Derivatives (Details)	3 Months Ended								12 Months Ended				12 Months Ended					12 Months Ended
	Dec. 31, 2011 USD ($)	Sep. 30, 2011 USD ($)	Jun. 30, 2011 USD ($)	Mar. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Sep. 30, 2010 USD ($)	Jun. 30, 2010 USD ($)	Mar. 31, 2010 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2011 JPY (¥)	Dec. 31, 2011 Fixed income/Interest rate USD ($)	Dec. 31, 2010 Fixed income/Interest rate USD ($)	Dec. 31, 2009 Fixed income/Interest rate USD ($)	Dec. 31, 2010 Foreign exchange USD ($)	Dec. 31, 2010 Equity USD ($)	Dec. 31, 2009 Weather Derivatives USD ($)	Dec. 31, 2011 Level 1 Inputs USD ($)	Dec. 31, 2011 Level 2 Inputs USD ($)	Dec. 31, 2011 Level 3 Inputs USD ($)	Dec. 31, 2011 Fair Value USD ($)	Dec. 31, 2011 Variable annuity reinsurance USD ($)	Dec. 31, 2010 Variable annuity reinsurance USD ($)	Dec. 31, 2009 Variable annuity reinsurance USD ($)	Dec. 31, 2011 Variable annuity reinsurance Level 1 Inputs Foreign exchange Cash Citigroup USD ($)	Dec. 31, 2010 Variable annuity reinsurance Level 1 Inputs Foreign exchange Cash Citigroup USD ($)	Dec. 31, 2009 Variable annuity reinsurance Level 1 Inputs Foreign exchange Cash Citigroup USD ($)	Dec. 31, 2011 Variable annuity reinsurance Level 1 Inputs Equity Cash Citigroup USD ($)	Dec. 31, 2010 Variable annuity reinsurance Level 1 Inputs Equity Cash Citigroup USD ($)	Dec. 31, 2009 Variable annuity reinsurance Level 1 Inputs Equity Cash Citigroup USD ($)	Dec. 31, 2011 WM Life Re USD ($)	Dec. 31, 2010 WM Life Re USD ($)	Dec. 31, 2009 WM Life Re USD ($)	Dec. 31, 2011 WM Life Re Citigroup USD ($)	Dec. 31, 2011 WM Life Re Royal Bank of Scotland USD ($)	Dec. 31, 2011 WM Life Re Bank of America USD ($)	Dec. 31, 2011 WM Life Re JP Morgan Chase USD ($)	Dec. 31, 2011 WM Life Re Nomura USD ($)	Dec. 31, 2011 WM Life Re Barclays USD ($)	Dec. 31, 2011 WM Life Re Goldman Sachs USD ($)	Dec. 31, 2011 WM Life Re Cash USD ($)	Dec. 31, 2010 WM Life Re Cash USD ($)	Dec. 31, 2011 WM Life Re Fixed maturity investments. USD ($)	Dec. 31, 2010 WM Life Re Fixed maturity investments. USD ($)	Dec. 31, 2011 WM Life Re Short-term investments USD ($)	Dec. 31, 2010 WM Life Re Short-term investments USD ($)
Variable annuity reinsurance
Total guarantee value	$ 3,000,000,000								$ 3,000,000,000			¥ 233,700,000,000
Collective account values of underlying variable annuities as a percentage of the guarantee value	78.00%								78.00%
Pre-tax operating results of WM Life Re
Fees, included in other revenues																																32,500,000	30,200,000	28,500,000
Change in fair value of variable annuity liability, included in other revenues																							7,200,000	47,700,000	22,400,000							(156,500,000)	(223,500,000)	77,100,000
Change in fair value of derivatives, included in other revenues									92,900,000	127,000,000	(158,400,000)		8,900,000	17,600,000	8,000,000				10,700,000	67,700,000	14,500,000	92,900,000				29,500,000	144,600,000	(69,500,000)	54,500,000	(35,200,000)	(96,900,000)	92,900,000	127,000,000	(158,400,000)
Foreign exchange, included in other revenues																																15,100,000	21,400,000	(3,900,000)
Other investment income and gain (loss)																																(900,000)	(900,000)	(1,400,000)
Total revenues	593,600,000	501,300,000	553,600,000	529,600,000	552,800,000	551,000,000	670,800,000	792,100,000	2,178,100,000	2,566,700,000	3,443,300,000																					(16,900,000)	(45,800,000)	(58,100,000)
Change in fair value of variable annuity death benefit liabilities, included in other expenses																																(1,800,000)	(6,000,000)	9,300,000
Death benefit claims paid, included in other expenses																																(3,800,000)	(2,700,000)	(2,200,000)
General and administrative expenses									(175,300,000)	(155,100,000)	(197,400,000)																					(4,700,000)	(6,300,000)	(5,600,000)
Pre-tax loss	58,000,000	1,200,000	36,100,000	(33,700,000)	95,600,000	75,200,000	5,900,000	(23,600,000)	61,600,000	153,100,000	797,200,000																					(27,200,000)	(60,800,000)	(56,600,000)
Changes in variable annuity liabilities
Balance of liabilities at the beginning of the period																					(610,200,000)
Realized and unrealized gains (losses)																					(158,300,000)
Balance of liabilities at the end of the period																					(768,500,000)
Changes in derivative instruments
Balance of assets at the beginning of the period				347,500,000					347,500,000				43,900,000			225,300,000	78,300,000			72,200,000	275,300,000	347,500,000
Purchases																					5,000,000	5,000,000
Realized and unrealized gains (losses)									92,900,000	127,000,000	(158,400,000)		8,900,000	17,600,000	8,000,000				10,700,000	67,700,000	14,500,000	92,900,000				29,500,000	144,600,000	(69,500,000)	54,500,000	(35,200,000)	(96,900,000)	92,900,000	127,000,000	(158,400,000)
Sales/settlements																			(6,600,000)	(100,700,000)	(47,700,000)	(155,000,000)
Balance of assets at the end of the period	290,400,000				347,500,000				290,400,000	347,500,000			31,100,000	43,900,000		225,300,000	78,300,000		4,100,000	39,200,000	247,100,000	290,400,000				161,300,000			98,000,000
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized derivative gains (losses) recognized in other revenues									92,900,000	127,000,000	(158,400,000)		8,900,000	17,600,000	8,000,000				10,700,000	67,700,000	14,500,000	92,900,000				29,500,000	144,600,000	(69,500,000)	54,500,000	(35,200,000)	(96,900,000)	92,900,000	127,000,000	(158,400,000)
Carrying value	290,400,000				347,500,000				290,400,000	347,500,000			31,100,000	43,900,000		225,300,000	78,300,000		4,100,000	39,200,000	247,100,000	290,400,000				161,300,000			98,000,000
Credit risk on derivative instruments
OTC derivative instruments																																295,400,000	351,500,000
Collateral held																																73,200,000	101,000,000									(453,500,000)	(291,100,000)	(31,200,000)	(3,300,000)	(600,000)	(31,600,000)
Collateral provided																																83,000,000	25,300,000												(48,600,000)	(73,200,000)	(52,400,000)
Cash and short-term investments held and posted as collateral to its reinsurance counterparties																																485,300,000	326,000,000
Net exposure to credit losses on fair value of OTC instruments																																305,200,000	275,800,000		67,700,000	82,500,000	54,200,000	34,200,000	31,900,000	25,100,000	9,600,000
Number of creditworthiness ratings																																21
Adjustments for counterparty credit risk																																(9,100,000)	(4,000,000)
Weather Derivatives
Net losses recognized on weather derivatives and weather contingent derivatives portfolio																		$ 2,600,000