Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Notional Amount	$ 12,720,346	$ 11,928,989
Carrying Value / Fair Value of Assets	327,453	301,270
Derivative Liability, Fair Value, Gross Liability	1,332,333	1,202,203
Not Designated as Hedging Instrument
Derivative [Line Items]
Notional Amount	11,434,761	10,985,503
Carrying Value / Fair Value of Assets	214,887	155,005
Derivative Liability, Fair Value, Gross Liability	1,303,005	1,172,217
Not Designated as Hedging Instrument | Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount	1,164,929	1,123,057
Carrying Value / Fair Value of Assets	142,357	85,075
Derivative Liability, Fair Value, Gross Liability	1,507	4,196
Not Designated as Hedging Instrument | Financial Futures
Derivative [Line Items]
Notional Amount	442,723	420,665
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	38,000	45,000
Carrying Value / Fair Value of Assets	915	44
Derivative Liability, Fair Value, Gross Liability	3,312	6,768
Not Designated as Hedging Instrument | Consumer Price Index Swap [Member]
Derivative [Line Items]
Notional Amount	30,015	28,561
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	402	292
Not Designated as Hedging Instrument | Credit Default Swaps
Derivative [Line Items]
Notional Amount	855,700	897,000
Carrying Value / Fair Value of Assets	8,709	8,230
Derivative Liability, Fair Value, Gross Liability	7,724	11,053
Not Designated as Hedging Instrument | Equity Options
Derivative [Line Items]
Notional Amount	442,335	453,435
Carrying Value / Fair Value of Assets	47,100	46,653
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument | Longevity Swap [Member]
Derivative [Line Items]
Notional Amount	910,400	868,960
Carrying Value / Fair Value of Assets	15,806	15,003
Derivative Liability, Fair Value, Gross Liability	0	7
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	3,043	2,619
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	0
Derivative Liability, Fair Value, Gross Liability	168,948	76,698
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	862,658	878,114
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	255,411	192,470
Not Designated as Hedging Instrument | Synthetic Guaranteed Investment Contracts
Derivative [Line Items]
Notional Amount	7,500,659	7,098,825
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	1,285,585	943,486
Carrying Value / Fair Value of Assets	112,566	146,265
Derivative Liability, Fair Value, Gross Liability	29,328	29,986
Designated As Hedging Instrument | Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount	435,000	120,000
Carrying Value / Fair Value of Assets	5,129	0
Derivative Liability, Fair Value, Gross Liability	26,724	29,986
Designated As Hedging Instrument | Currency Swap [Member]
Derivative [Line Items]
Notional Amount	850,585	823,486
Carrying Value / Fair Value of Assets	107,437	146,265
Derivative Liability, Fair Value, Gross Liability	$ 2,604	$ 0