Investments (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Pledged Assets
Restricted cash	$ 21,300,000	$ 271,200,000
Restricted securities	2,314,700,000	1,679,200,000
Interest Rate Disclosures [Abstract]
Low end of interest rate on funds held	2.00%	3.00%
High end of interest rate on funds held	5.00%	6.00%
[TradingSecuritiesMember]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	14,354,580,000	13,386,674,000
Gross unrealized gains	723,788,000	638,313,000
Gross unrealized losses	(149,277,000)	(79,525,000)
Fair value	14,929,091,000	13,945,462,000
Net Payable/Receivable for Securites Purchased/Sold
Receivable for securities sold	71,477,000	16,669,000
Payable for securities purchased	(86,557,000)	(23,802,000)
Net (payable)/receivable for securities purchased/sold	(15,080,000)	(7,133,000)
[TradingSecuritiesMember] | [CostMember]
Maturity Profile [Line Items]
One year or less	566,515,000
More than one year through five years	4,792,769,000
More than five years through ten years	3,471,876,000
More than ten years	682,305,000
Subtotal	9,513,465,000
Mortgage/asset-backed securities	3,923,502,000
Total	13,436,967,000
[TradingSecuritiesMember] | [FairValueMember]
Maturity Profile [Line Items]
One year or less	570,829,000
More than one year through five years	4,923,116,000
More than five years through ten years	3,713,242,000
More than ten years	785,933,000
Subtotal	9,993,120,000
Mortgage/asset-backed securities	3,991,280,000
Total	13,984,400,000
[TradingSecuritiesMember] | [FixedMaturitiesMember]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	13,394,404,000	12,394,797,000
Gross unrealized gains	624,614,000	491,330,000
Gross unrealized losses	(77,189,000)	(61,738,000)
Fair value	13,941,829,000	12,824,389,000
[TradingSecuritiesMember] | [FixedMaturitiesMember] | [USTreasuryAndGovernmentMember]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	1,084,533,000	895,862,000
Gross unrealized gains	31,283,000	15,584,000
Gross unrealized losses	(39,000)	(5,414,000)
Fair value	1,115,777,000	906,032,000
[TradingSecuritiesMember] | [FixedMaturitiesMember] | [USStatesAndPoliticalSubdivisionsMember]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	117,528,000	66,548,000
Gross unrealized gains	6,169,000	271,000
Gross unrealized losses	(13,000)	(127,000)
Fair value	123,684,000	66,692,000
[TradingSecuritiesMember] | [FixedMaturitiesMember] | [NonUsSovereignGovernmentSupranationalAndGovernmentRelatedDebtSecuritiesMember]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	2,807,363,000	2,743,922,000
Gross unrealized gains	158,900,000	82,984,000
Gross unrealized losses	(2,172,000)	(7,713,000)
Fair value	2,964,091,000	2,819,193,000
[TradingSecuritiesMember] | [FixedMaturitiesMember] | [CorporateDebtSecuritiesMember]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	5,461,478,000	5,876,392,000
Gross unrealized gains	319,090,000	286,463,000
Gross unrealized losses	(33,571,000)	(19,008,000)
Fair value	5,746,997,000	6,143,847,000
[TradingSecuritiesMember] | [FixedMaturitiesMember] | [AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	626,508,000	554,326,000
Gross unrealized gains	11,558,000	11,707,000
Gross unrealized losses	(4,267,000)	(9,376,000)
Fair value	633,799,000	556,657,000
[TradingSecuritiesMember] | [FixedMaturitiesMember] | [ResidentialMortgageBackedSecuritiesMember]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	3,224,850,000	2,227,938,000
Gross unrealized gains	94,781,000	92,534,000
Gross unrealized losses	(36,730,000)	(14,947,000)
Fair value	3,282,901,000	2,305,525,000
[TradingSecuritiesMember] | [FixedMaturitiesMember] | [CommercialMortgageBackedSecuritiesMember]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	72,144,000	29,809,000
Gross unrealized gains	2,833,000	1,787,000
Gross unrealized losses	(397,000)	(5,153,000)
Fair value	74,580,000	26,443,000
[TradingSecuritiesMember] | [ShortTermInvestmentsMember]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	42,563,000	49,132,000
Gross unrealized gains	22,000	283,000
Gross unrealized losses	(14,000)	(18,000)
Fair value	42,571,000	49,397,000
[TradingSecuritiesMember] | [EquitySecuritiesMember]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	917,613,000	942,745,000
Gross unrealized gains	99,152,000	146,700,000
Gross unrealized losses	(72,074,000)	(17,769,000)
Fair value	$ 944,691,000	$ 1,071,676,000