Derivative Instruments (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Derivative Instruments [Line Items]
Notional Amount of Derivatives	$ 6,598,107,000	$ 4,989,001,000
Derivative Asset, Fair Value, Gross Asset	190,634,000	286,951,000
Derivative Liability, Fair Value, Gross Liability	1,209,616,000	1,444,304,000
Designated As Hedging [Member]
Derivative Instruments [Line Items]
Notional Amount of Derivatives	686,787,000	677,828,000
Derivative Asset, Fair Value, Gross Asset	344,000	419,000
Derivative Liability, Fair Value, Gross Liability	28,184,000	24,956,000
Designated As Non Hedging [Member]
Derivative Instruments [Line Items]
Notional Amount of Derivatives	5,911,320,000	4,311,173,000
Derivative Asset, Fair Value, Gross Asset	190,290,000	286,532,000
Derivative Liability, Fair Value, Gross Liability	1,181,432,000	1,419,348,000
Interest Rate Swap [Member] | Designated As Hedging [Member]
Derivative Instruments [Line Items]
Notional Amount of Derivatives	57,275,000	56,250,000
Derivative Asset, Fair Value, Gross Asset	344,000	133,000
Derivative Liability, Fair Value, Gross Liability	786,000	960,000
Interest Rate Swap [Member] | Designated As Non Hedging [Member]
Derivative Instruments [Line Items]
Notional Amount of Derivatives	2,195,059,000	2,748,317,000
Derivative Asset, Fair Value, Gross Asset	123,085,000	184,842,000
Derivative Liability, Fair Value, Gross Liability	17,867,000	18,702,000
Future [Member] | Designated As Non Hedging [Member]
Derivative Instruments [Line Items]
Notional Amount of Derivatives	127,877,000	277,814,000
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	0	0
Foreign Exchange Forward [Member] | Designated As Non Hedging [Member]
Derivative Instruments [Line Items]
Notional Amount of Derivatives	74,400,000	24,400,000
Derivative Asset, Fair Value, Gross Asset	1,017,000	4,560,000
Derivative Liability, Fair Value, Gross Liability	2,105,000	0
Consumer Price Index Swap [Member] | Designated As Non Hedging [Member]
Derivative Instruments [Line Items]
Notional Amount of Derivatives	85,135,000	101,069,000
Derivative Asset, Fair Value, Gross Asset	1,446,000	766,000
Derivative Liability, Fair Value, Gross Liability	0	0
Credit Default Swap [Member] | Designated As Non Hedging [Member]
Derivative Instruments [Line Items]
Notional Amount of Derivatives	714,000,000	649,500,000
Derivative Asset, Fair Value, Gross Asset	2,228,000	1,313,000
Derivative Liability, Fair Value, Gross Liability	5,922,000	10,949,000
Equity [Member] | Designated As Non Hedging [Member]
Derivative Instruments [Line Items]
Notional Amount of Derivatives	696,776,000	510,073,000
Derivative Asset, Fair Value, Gross Asset	62,514,000	90,106,000
Derivative Liability, Fair Value, Gross Liability	0	0
Currency Swap [Member] | Designated As Hedging [Member]
Derivative Instruments [Line Items]
Notional Amount of Derivatives	629,512,000	621,578,000
Derivative Asset, Fair Value, Gross Asset	0	286,000
Derivative Liability, Fair Value, Gross Liability	27,398,000	23,996,000
Embedded Derivatives In Modified Coinsurance Or Funds Withheld Arrangements [Member] | Designated As Non Hedging [Member]
Derivative Instruments [Line Items]
Notional Amount of Derivatives	0	0
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	243,177,000	361,456,000
Embedded Derivatives In Indexed Annuity [Member] | Designated As Non Hedging [Member]
Derivative Instruments [Line Items]
Notional Amount of Derivatives	0	0
Derivative Asset, Fair Value, Gross Asset	0	4,945,000
Derivative Liability, Fair Value, Gross Liability	740,256,000	751,523,000
Embedded Derivatives In Variable Annuity [Member] | Designated As Non Hedging [Member]
Derivative Instruments [Line Items]
Notional Amount of Derivatives	0	0
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	172,105,000	276,718,000
Synthetic Guaranteed Investment Contracts [Member] | Designated As Non Hedging [Member]
Derivative Instruments [Line Items]
Notional Amount of Derivatives	2,018,073,000	0
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	$ 0	$ 0