Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional Amount	$ 14,561,937	$ 14,012,598
Carrying Value / Fair Value of Assets	281,527	283,094
Derivative Liability, Fair Value, Gross Liability	1,015,305	1,061,408
Not Designated as Hedging Instrument
Derivative [Line Items]
Notional Amount	13,079,853	12,649,093
Carrying Value / Fair Value of Assets	193,942	150,834
Derivative Liability, Fair Value, Gross Liability	981,851	1,029,451
Not Designated as Hedging Instrument | Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount	973,825	949,556
Carrying Value / Fair Value of Assets	63,370	78,405
Derivative Liability, Fair Value, Gross Liability	4,375	5,949
Not Designated as Hedging Instrument | Financial Futures
Derivative [Line Items]
Notional Amount	477,407	475,968
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	15,000	25,000
Carrying Value / Fair Value of Assets	21	0
Derivative Liability, Fair Value, Gross Liability	63	5,070
Not Designated as Hedging Instrument | Consumer Price Index Swap [Member]
Derivative [Line Items]
Notional Amount	21,991	20,615
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	187	262
Not Designated as Hedging Instrument | Credit Default Swaps
Derivative [Line Items]
Notional Amount	945,000	926,000
Carrying Value / Fair Value of Assets	7,620	12,012
Derivative Liability, Fair Value, Gross Liability	1,043	2,871
Not Designated as Hedging Instrument | Equity Options
Derivative [Line Items]
Notional Amount	541,532	525,894
Carrying Value / Fair Value of Assets	28,301	33,459
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument | Longevity Swap [Member]
Derivative [Line Items]
Notional Amount	914,080	841,360
Carrying Value / Fair Value of Assets	33,349	26,958
Derivative Liability, Fair Value, Gross Liability	0	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	1,552	2,462
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	61,281	0
Derivative Liability, Fair Value, Gross Liability	0	22,529
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	812,718	805,672
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	161,913	184,636
Not Designated as Hedging Instrument | Synthetic Guaranteed Investment Contracts
Derivative [Line Items]
Notional Amount	9,141,018	8,834,700
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	1,482,084	1,363,505
Carrying Value / Fair Value of Assets	87,585	132,260
Derivative Liability, Fair Value, Gross Liability	33,454	31,957
Designated As Hedging Instrument | Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount	435,000	435,000
Carrying Value / Fair Value of Assets	1,267	27,901
Derivative Liability, Fair Value, Gross Liability	22,120	31,223
Designated As Hedging Instrument | Foreign Currency Forwards
Derivative [Line Items]
Notional Amount	150,211	0
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	4,158	0
Designated As Hedging Instrument | Currency Swap [Member]
Derivative [Line Items]
Notional Amount	896,873	928,505
Carrying Value / Fair Value of Assets	86,318	104,359
Derivative Liability, Fair Value, Gross Liability	$ 7,176	$ 734