Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional Amount	$ 14,700,117	$ 14,012,598
Carrying Value / Fair Value of Assets	284,281	283,094
Derivative Liability, Fair Value, Gross Liability	1,040,570	1,061,408
Not Designated as Hedging Instrument
Derivative [Line Items]
Notional Amount	13,137,493	12,649,093
Carrying Value / Fair Value of Assets	211,831	150,834
Derivative Liability, Fair Value, Gross Liability	994,358	1,029,451
Not Designated as Hedging Instrument | Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount	946,726	949,556
Carrying Value / Fair Value of Assets	61,263	78,405
Derivative Liability, Fair Value, Gross Liability	1,825	5,949
Not Designated as Hedging Instrument | Financial Futures
Derivative [Line Items]
Notional Amount	448,600	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	6,000	25,000
Carrying Value / Fair Value of Assets	(84)	0
Derivative Liability, Fair Value, Gross Liability	0	5,070
Not Designated as Hedging Instrument | Consumer Price Index Swap [Member]
Derivative [Line Items]
Notional Amount	132,081	20,615
Carrying Value / Fair Value of Assets	589	0
Derivative Liability, Fair Value, Gross Liability	420	262
Not Designated as Hedging Instrument | Credit Default Swaps
Derivative [Line Items]
Notional Amount	948,000	926,000
Carrying Value / Fair Value of Assets	7,905	12,012
Derivative Liability, Fair Value, Gross Liability	490	2,871
Not Designated as Hedging Instrument | Equity Options
Derivative [Line Items]
Notional Amount	541,532	525,894
Carrying Value / Fair Value of Assets	20,006	33,459
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument | Longevity Swap [Member]
Derivative [Line Items]
Notional Amount	945,120	841,360
Carrying Value / Fair Value of Assets	37,827	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	1,683	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	84,325	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	821,821	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	168,119	184,636
Not Designated as Hedging Instrument | Synthetic Guaranteed Investment Contracts
Derivative [Line Items]
Notional Amount	9,119,434	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	1,562,624	1,363,505
Carrying Value / Fair Value of Assets	72,450	132,260
Derivative Liability, Fair Value, Gross Liability	46,212	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	0	27,901
Derivative Liability, Fair Value, Gross Liability	22,133	31,223
Designated As Hedging Instrument | Foreign Currency Forwards
Derivative [Line Items]
Notional Amount	331,135	0
Carrying Value / Fair Value of Assets	1,418	0
Derivative Liability, Fair Value, Gross Liability	10,203	0
Designated As Hedging Instrument | Currency Swap [Member]
Derivative [Line Items]
Notional Amount	796,489	928,505
Carrying Value / Fair Value of Assets	71,032	104,359
Derivative Liability, Fair Value, Gross Liability	$ 13,876	$ 734