Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Notional Amount of Derivatives	$ 14,012,598	$ 11,928,989
Derivative Asset, Fair Value, Gross Asset	283,094	301,270
Derivative Liability, Fair Value, Gross Liability	1,061,408	1,202,203
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount of Derivatives	12,649,093	10,985,503
Derivative Asset, Fair Value, Gross Asset	150,834	155,005
Derivative Liability, Fair Value, Gross Liability	1,029,451	1,172,217
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	949,556	1,123,057
Derivative Asset, Fair Value, Gross Asset	78,405	85,075
Derivative Liability, Fair Value, Gross Liability	5,949	4,196
Not Designated as Hedging Instrument [Member] | Future [Member]
Derivative [Line Items]
Notional Amount of Derivatives	475,968	420,665
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	45,000
Derivative Asset, Fair Value, Gross Asset	0	44
Derivative Liability, Fair Value, Gross Liability	5,070	6,768
Not Designated as Hedging Instrument [Member] | Consumer Price Index Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	20,615	28,561
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	262	292
Not Designated as Hedging Instrument [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	926,000	897,000
Derivative Asset, Fair Value, Gross Asset	12,012	8,230
Derivative Liability, Fair Value, Gross Liability	2,871	11,053
Not Designated as Hedging Instrument [Member] | Equity [Member]
Derivative [Line Items]
Notional Amount of Derivatives	525,894	453,435
Derivative Asset, Fair Value, Gross Asset	33,459	46,653
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument [Member] | Longevity Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	841,360	868,960
Derivative Asset, Fair Value, Gross Asset	26,958	15,003
Derivative Liability, Fair Value, Gross Liability	0	7
Not Designated as Hedging Instrument [Member] | Mortality Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	50,000	50,000
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	2,462	2,619
Not Designated as Hedging Instrument [Member] | Synthetic Guaranteed Investment Contracts [Member]
Derivative [Line Items]
Notional Amount of Derivatives	8,834,700	7,098,825
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	0	0
Derivative Liability, Fair Value, Gross Liability	22,529	76,698
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	805,672	878,114
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	184,636	192,470
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount of Derivatives	1,363,505	943,486
Derivative Asset, Fair Value, Gross Asset	132,260	146,265
Derivative Liability, Fair Value, Gross Liability	31,957	29,986
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	435,000	120,000
Derivative Asset, Fair Value, Gross Asset	27,901	0
Derivative Liability, Fair Value, Gross Liability	31,223	29,986
Designated as Hedging Instrument [Member] | Currency Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	928,505	823,486
Derivative Asset, Fair Value, Gross Asset	104,359	146,265
Derivative Liability, Fair Value, Gross Liability	$ 734	$ 0