Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional Amount of Derivatives	$ 19,884,691	$ 15,904,127
Derivative Asset, Fair Value, Gross Asset	356,666	320,724
Derivative Liability, Fair Value, Gross Liability	990,895	1,058,727
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount of Derivatives	18,044,033	14,243,031
Derivative Asset, Fair Value, Gross Asset	254,381	254,252
Derivative Liability, Fair Value, Gross Liability	963,638	1,022,122
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	1,040,588	996,204
Derivative Asset, Fair Value, Gross Asset	47,652	59,809
Derivative Liability, Fair Value, Gross Liability	961	2,372
Not Designated as Hedging Instrument [Member] | Future [Member]
Derivative [Line Items]
Notional Amount of Derivatives	325,620	412,438
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument [Member] | Foreign Exchange Forward [Member]
Derivative [Line Items]
Notional Amount of Derivatives	25,000	6,030
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	234	28
Not Designated as Hedging Instrument [Member] | Consumer Price Index Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	385,580	221,932
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	11,384	2,160
Not Designated as Hedging Instrument [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	1,338,300	961,200
Derivative Asset, Fair Value, Gross Asset	6,003	8,319
Derivative Liability, Fair Value, Gross Liability	1,166	1,651
Not Designated as Hedging Instrument [Member] | Equity [Member]
Derivative [Line Items]
Notional Amount of Derivatives	439,158	632,251
Derivative Asset, Fair Value, Gross Asset	42,836	23,271
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument [Member] | Longevity Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	917,360	960,400
Derivative Asset, Fair Value, Gross Asset	47,789	40,659
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument [Member] | Mortality Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	25,000	0
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	369	1,683
Not Designated as Hedging Instrument [Member] | Synthetic Guaranteed Investment Contracts [Member]
Derivative [Line Items]
Notional Amount of Derivatives	13,397,729	10,052,576
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument [Member] | Embedded Derivatives In Modified Coinsurance Or Funds Withheld Arrangements [Member]
Derivative [Line Items]
Notional Amount of Derivatives	0	0
Derivative Asset, Fair Value, Gross Asset	109,597	122,194
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument [Member] | Embedded Derivatives In Indexed Annuity [Member]
Derivative [Line Items]
Notional Amount of Derivatives	0	0
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	776,940	861,758
Not Designated as Hedging Instrument [Member] | Embedded Derivatives In Variable Annuity [Member]
Derivative [Line Items]
Notional Amount of Derivatives	0	0
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	167,925	152,470
Not Designated as Hedging Instrument [Member] | Currency Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	149,698	0
Derivative Asset, Fair Value, Gross Asset	504	0
Derivative Liability, Fair Value, Gross Liability	4,659	0
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount of Derivatives	1,840,658	1,661,096
Derivative Asset, Fair Value, Gross Asset	102,285	66,472
Derivative Liability, Fair Value, Gross Liability	27,257	36,605
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	435,000	435,000
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	27,257	20,389
Designated as Hedging Instrument [Member] | Foreign Exchange Forward [Member]
Derivative [Line Items]
Notional Amount of Derivatives	911,197	553,175
Derivative Asset, Fair Value, Gross Asset	50,974	1,265
Derivative Liability, Fair Value, Gross Liability	0	7,720
Designated as Hedging Instrument [Member] | Currency Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	494,461	672,921
Derivative Asset, Fair Value, Gross Asset	51,311	65,207
Derivative Liability, Fair Value, Gross Liability	$ 0	$ 8,496