Derivative Instruments (Non Hedging Derivatives and Embedded Derivatives Effect on Income Statement) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	$ 12,472	$ (10,572)	$ 20,260	$ (11,042)
Policy acquisition costs and other insurance expenses	(260,345)	(320,276)	(572,226)	(677,178)
Interest credited	157,842	109,327	291,031	189,776
Other revenues	107,072	83,959	201,625	159,256
Gain Loss On Investments Related To Change In Fair Value Of Derivatives Excluding Embedded Derivatives	13,361	(14,151)	2,238	(33,625)
Not Designated as Hedging Instrument
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	(7,880)	16,497	136	50,770
Not Designated as Hedging Instrument | Interest Rate Swap [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	33,404	(8,600)	57,378	(35,171)
Not Designated as Hedging Instrument | Financial Futures
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	(7,685)	(897)	(29,963)	(768)
Not Designated as Hedging Instrument | Currency Swap [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	(6,380)	0	(5,657)	0
Not Designated as Hedging Instrument | Foreign Currency Forwards
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	(258)	(262)	234	61
Not Designated as Hedging Instrument | Consumer Price Index Swap [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	(6,812)	1,041	(15,663)	3,227
Not Designated as Hedging Instrument | Credit Default Swaps
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	5,078	1,084	19,578	682
Not Designated as Hedging Instrument | Equity Options
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	(4,828)	(8,007)	(27,512)	(5,414)
Not Designated as Hedging Instrument | Longevity Swap [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Other revenues	2,184	2,289	4,327	4,557
Not Designated as Hedging Instrument | Mortality Swap [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Other revenues	(1,342)	(799)	(484)	(799)
Not Designated as Hedging Instrument | Embedded Derivatives In Modified Coinsurance Or Funds Withheld Arrangements
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	5,262	8,805	3,170	22,416
Not Designated as Hedging Instrument | Embedded Derivatives In Indexed Annuity [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Interest credited	(8,459)	(6,519)	(5,389)	(31,870)
Not Designated as Hedging Instrument | Embedded Derivatives In Variable Annuity [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	$ (18,044)	$ 15,324	$ 117	$ 30,109