Claim and Claim Adjustment Expense Reserves (Narrative) (Details) (USD $)	12 Months Ended							12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Loss Portfolio Transfer [Member]	Dec. 31, 2010 Loss Portfolio Transfer [Member]	Aug. 31, 2010 Loss Portfolio Transfer [Member]	Dec. 31, 2009 Loss Portfolio Transfer [Member]	Dec. 31, 2009 Willis Limited [Member]
Catastrophe losses net of reinsurance	$ 222,000,000	$ 121,000,000	$ 89,000,000
Net A&EP claim and allocated claim adjustment expense reserves ceded to NICO							1,600,000,000
Aggregate limit under A&EP Loss Portfolio Transfer				4,000,000,000
Net claim and allocated claim adjustment expense reserves in discontinued operations ceded to NICO under A&EP Loss Portfolio Transfer							90,000,000
A&EP claim and allocated claim adjustment expense reserves ceded under existing third party reinsurance contracts transferred to NICO under A&EP Loss Portfolio Transfer							1,200,000,000
Reinsurance premium paid to NICO under A&EP Loss Portfolio Transfer						2,000,000,000
Net reinsurance receivables transferrred to NICO under A&EP Loss Portfolio Transfer							215,000,000
Allowance for doubtful accounts related to reinsurance receivables transferred to NICO under A&EP Loss Portfolio Transfer							100,000,000
Collateral trust established by NICO under A&EP Loss Portfolio Transfer						2,200,000,000
Reduction of allowance for uncollectible reinsurance receivables					200,000,000
Gross A&EP claim and allocated claim adjustment expense reserves ceded under the Loss Portfolio Transfer and other existing third party reinsurance agreements				2,300,000,000	2,500,000,000
Remaining amount available under aggregate limit of the Loss Portfolio Transfer on an incurred basis				2,300,000,000
Net ultimate paid losses ceded under the Loss Portfolio Transfer				351,000,000
(Unfavorable) favorable net prior year development, Life & Group	29,000,000	2,000,000	53,000,000
(Unfavorable) favorable net prior year development, P&C	423,000,000	637,000,000	289,000,000	(138,000,000)
Settlement agreement, amount								130,000,000
Loss recovery, net of reinsurance								$ 94,000,000