Derivative Instruments (Non Hedging Derivatives and Embedded Derivatives Effect on Income Statement) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	$ 167,880	$ 94,195	$ (164,750)
Other revenues	(352,108)	(266,559)	(277,692)
Gain Loss On Investments Related To Change In Fair Value Of Derivatives Excluding Embedded Derivatives	(42,767)	(28,611)	7,408
Interest credited	502,040	364,691	336,964
Not Designated as Hedging Instrument [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	54,060	44,101	(105,136)
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	11,278	7,649	20,358
Not Designated as Hedging Instrument [Member] | Interest Rate Options [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	0	0	3,275
Not Designated as Hedging Instrument [Member] | Future [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	(36,160)	(40,242)	319
Not Designated as Hedging Instrument [Member] | Foreign Exchange Forward [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	591	1,630	(1,160)
Not Designated as Hedging Instrument [Member] | Consumer Price Index Swap [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	(2,078)	(401)	(208)
Not Designated as Hedging Instrument [Member] | Credit Default Swap [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	18,118	18,100	(4,683)
Not Designated as Hedging Instrument [Member] | Equity [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	(42,953)	(28,270)	(16,899)
Not Designated as Hedging Instrument [Member] | Longevity Swap [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Other revenues	(9,358)	(13,095)	(8,228)
Not Designated as Hedging Instrument [Member] | Mortality Swap [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Other revenues	(921)	(172)	(1,822)
Not Designated as Hedging Instrument [Member] | Embedded Derivatives In Modified Coinsurance Or Funds Withheld Arrangements [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	144,723	54,169	(98,792)
Not Designated as Hedging Instrument [Member] | Embedded Derivatives In Indexed Annuity [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Interest credited	(80,062)	10,708	19,440
Not Designated as Hedging Instrument [Member] | Embedded Derivatives In Variable Annuity [Member]
Income Affect Of Non Hedging Derivatives [Line Items]
Total investment related gains (losses), net	$ 32,166	$ 7,835	$ (33,192)