Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Notional Amount	$ 11,808,397	$ 11,125,976
Carrying Value / Fair Value of Assets	310,405	260,898
Derivative Liability, Fair Value, Gross Liability	1,216,450	1,141,831
Not Designated as Hedging Instrument
Derivative [Line Items]
Notional Amount	10,861,506	10,132,552
Carrying Value / Fair Value of Assets	181,919	188,817
Derivative Liability, Fair Value, Gross Liability	1,177,221	1,109,058
Not Designated as Hedging Instrument | Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount	1,130,627	1,144,661
Carrying Value / Fair Value of Assets	101,194	93,783
Derivative Liability, Fair Value, Gross Liability	3,130	3,934
Not Designated as Hedging Instrument | Interest Rate Options
Derivative [Line Items]
Notional Amount	0	240,000
Carrying Value / Fair Value of Assets	0	18,195
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument | Financial Futures
Derivative [Line Items]
Notional Amount	354,914	275,983
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument | Foreign Currency Forwards
Derivative [Line Items]
Notional Amount	40,000	67,967
Carrying Value / Fair Value of Assets	343	87
Derivative Liability, Fair Value, Gross Liability	6,675	15,098
Not Designated as Hedging Instrument | Consumer Price Index Swaps
Derivative [Line Items]
Notional Amount	36,002	41,938
Carrying Value / Fair Value of Assets	14	0
Derivative Liability, Fair Value, Gross Liability	282	561
Not Designated as Hedging Instrument | Credit Default Swaps
Derivative [Line Items]
Notional Amount	902,000	805,700
Carrying Value / Fair Value of Assets	7,015	11,689
Derivative Liability, Fair Value, Gross Liability	9,007	3,502
Not Designated as Hedging Instrument | Equity Options
Derivative [Line Items]
Notional Amount	483,830	555,361
Carrying Value / Fair Value of Assets	60,032	35,242
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument | Longevity Swap [Member]
Derivative [Line Items]
Notional Amount	894,160	450,000
Carrying Value / Fair Value of Assets	13,321	7,727
Derivative Liability, Fair Value, Gross Liability	23	0
Not Designated as Hedging Instrument | Mortality Swap [Member]
Derivative [Line Items]
Notional Amount	50,000	50,000
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	2,196	797
Not Designated as Hedging Instrument | Embedded Derivatives In Modified Coinsurance Or Funds Withheld Arrangements
Derivative [Line Items]
Notional Amount	0	0
Carrying Value / Fair Value of Assets	0	22,094
Derivative Liability, Fair Value, Gross Liability	49,498	0
Not Designated as Hedging Instrument | Embedded Derivatives In Indexed Annuity [Member]
Derivative [Line Items]
Notional Amount	0	0
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	877,503	925,887
Not Designated as Hedging Instrument | Embedded Derivatives In Variable Annuity [Member]
Derivative [Line Items]
Notional Amount	0	0
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	228,907	159,279
Not Designated as Hedging Instrument | Synthetic Guaranteed Investment Contracts
Derivative [Line Items]
Notional Amount	6,969,973	6,500,942
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	0	0
Designated As Hedging Instrument
Derivative [Line Items]
Notional Amount	946,891	993,424
Carrying Value / Fair Value of Assets	128,486	72,081
Derivative Liability, Fair Value, Gross Liability	39,229	32,773
Designated As Hedging Instrument | Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount	120,000	120,000
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	36,801	18,228
Designated As Hedging Instrument | Currency Swap [Member]
Derivative [Line Items]
Notional Amount	826,891	676,972
Carrying Value / Fair Value of Assets	128,486	70,906
Derivative Liability, Fair Value, Gross Liability	2,428	0
Designated As Hedging Instrument | Forward Contracts [Member]
Derivative [Line Items]
Notional Amount	0	196,452
Carrying Value / Fair Value of Assets	0	1,175
Derivative Liability, Fair Value, Gross Liability	$ 0	$ 14,545