Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Document Period End Date	Jun. 30, 2019
Notional Amount	$ 19,996,144	$ 19,884,691
Carrying Value / Fair Value of Assets	313,837	356,666
Derivative Liability, Fair Value, Gross Liability	981,238	990,895
Not Designated as Hedging Instrument
Derivative [Line Items]
Notional Amount	18,064,628	18,044,033
Carrying Value / Fair Value of Assets	257,080	254,381
Derivative Liability, Fair Value, Gross Liability	950,600	963,638
Not Designated as Hedging Instrument | Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount	922,478	1,040,588
Carrying Value / Fair Value of Assets	71,535	47,652
Derivative Liability, Fair Value, Gross Liability	3,447	961
Not Designated as Hedging Instrument | Financial Futures
Derivative [Line Items]
Notional Amount	389,353	325,620
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	0	25,000
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	0	234
Not Designated as Hedging Instrument | Consumer Price Index Swap [Member]
Derivative [Line Items]
Notional Amount	438,587	385,580
Carrying Value / Fair Value of Assets	189	0
Derivative Liability, Fair Value, Gross Liability	25,481	11,384
Not Designated as Hedging Instrument | Credit Default Swaps
Derivative [Line Items]
Notional Amount	1,331,300	1,338,300
Carrying Value / Fair Value of Assets	6,190	6,003
Derivative Liability, Fair Value, Gross Liability	134	1,166
Not Designated as Hedging Instrument | Equity Options
Derivative [Line Items]
Notional Amount	371,869	439,158
Carrying Value / Fair Value of Assets	14,644	42,836
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument | Longevity Swap [Member]
Derivative [Line Items]
Notional Amount	909,840	917,360
Carrying Value / Fair Value of Assets	51,755	47,789
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument | Mortality Swap [Member]
Derivative [Line Items]
Notional Amount	25,000	25,000
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	853	369
Not Designated as Hedging Instrument | Currency Swap [Member]
Derivative [Line Items]
Notional Amount	149,698	149,698
Carrying Value / Fair Value of Assets	0	504
Derivative Liability, Fair Value, Gross Liability	12,194	4,659
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	112,767	109,597
Derivative Liability, Fair Value, Gross Liability	0	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	740,683	776,940
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	167,808	167,925
Not Designated as Hedging Instrument | Synthetic Guaranteed Investment Contracts
Derivative [Line Items]
Notional Amount	13,526,503	13,397,729
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	1,931,516	1,840,658
Carrying Value / Fair Value of Assets	56,757	102,285
Derivative Liability, Fair Value, Gross Liability	30,638	27,257
Designated As Hedging Instrument | Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount	535,000	435,000
Carrying Value / Fair Value of Assets	825	0
Derivative Liability, Fair Value, Gross Liability	27,560	27,257
Designated As Hedging Instrument | Foreign Currency Forwards
Derivative [Line Items]
Notional Amount	987,606	911,197
Carrying Value / Fair Value of Assets	27,532	50,974
Derivative Liability, Fair Value, Gross Liability	653	0
Designated As Hedging Instrument | Currency Swap [Member]
Derivative [Line Items]
Notional Amount	408,910	494,461
Carrying Value / Fair Value of Assets	28,400	51,311
Derivative Liability, Fair Value, Gross Liability	$ 2,425	$ 0