Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional Amount of Derivatives	$ 15,904,127	$ 14,012,598
Derivative Asset, Fair Value, Gross Asset	320,724	283,094
Derivative Liability, Fair Value, Gross Liability	1,058,727	1,061,408
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount of Derivatives	14,243,031	12,649,093
Derivative Asset, Fair Value, Gross Asset	254,252	150,834
Derivative Liability, Fair Value, Gross Liability	1,022,122	1,029,451
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	996,204	949,556
Derivative Asset, Fair Value, Gross Asset	59,809	78,405
Derivative Liability, Fair Value, Gross Liability	2,372	5,949
Not Designated as Hedging Instrument [Member] | Future [Member]
Derivative [Line Items]
Notional Amount of Derivatives	412,438	475,968
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	6,030	25,000
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	28	5,070
Not Designated as Hedging Instrument [Member] | Consumer Price Index Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	221,932	20,615
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	2,160	262
Not Designated as Hedging Instrument [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	961,200	926,000
Derivative Asset, Fair Value, Gross Asset	8,319	12,012
Derivative Liability, Fair Value, Gross Liability	1,651	2,871
Not Designated as Hedging Instrument [Member] | Equity [Member]
Derivative [Line Items]
Notional Amount of Derivatives	632,251	525,894
Derivative Asset, Fair Value, Gross Asset	23,271	33,459
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument [Member] | Longevity Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	960,400	841,360
Derivative Asset, Fair Value, Gross Asset	40,659	26,958
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument [Member] | Mortality Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	0	50,000
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	1,683	2,462
Not Designated as Hedging Instrument [Member] | Synthetic Guaranteed Investment Contracts [Member]
Derivative [Line Items]
Notional Amount of Derivatives	10,052,576	8,834,700
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	122,194	0
Derivative Liability, Fair Value, Gross Liability	0	22,529
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	861,758	805,672
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	152,470	184,636
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount of Derivatives	1,661,096	1,363,505
Derivative Asset, Fair Value, Gross Asset	66,472	132,260
Derivative Liability, Fair Value, Gross Liability	36,605	31,957
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	27,901
Derivative Liability, Fair Value, Gross Liability	20,389	31,223
Designated as Hedging Instrument [Member] | Foreign Exchange Forward [Member]
Derivative [Line Items]
Notional Amount of Derivatives	553,175	0
Derivative Asset, Fair Value, Gross Asset	1,265	0
Derivative Liability, Fair Value, Gross Liability	7,720	0
Designated as Hedging Instrument [Member] | Currency Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	672,921	928,505
Derivative Asset, Fair Value, Gross Asset	65,207	104,359
Derivative Liability, Fair Value, Gross Liability	$ 8,496	$ 734