Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Notional Amount of Derivatives	$ 11,928,989	$ 11,125,976
Derivative Asset, Fair Value, Gross Asset	301,270	260,898
Derivative Liability, Fair Value, Gross Liability	1,202,203	1,141,831
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount of Derivatives	10,985,503	10,132,552
Derivative Asset, Fair Value, Gross Asset	155,005	188,817
Derivative Liability, Fair Value, Gross Liability	1,172,217	1,109,058
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	1,123,057	1,144,661
Derivative Asset, Fair Value, Gross Asset	85,075	93,783
Derivative Liability, Fair Value, Gross Liability	4,196	3,934
Not Designated as Hedging Instrument [Member] | Interest Rate Options [Member]
Derivative [Line Items]
Notional Amount of Derivatives	0	240,000
Derivative Asset, Fair Value, Gross Asset	0	18,195
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument [Member] | Future [Member]
Derivative [Line Items]
Notional Amount of Derivatives	420,665	275,983
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	45,000	67,967
Derivative Asset, Fair Value, Gross Asset	44	87
Derivative Liability, Fair Value, Gross Liability	6,768	15,098
Not Designated as Hedging Instrument [Member] | Consumer Price Index Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	28,561	41,938
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	292	561
Not Designated as Hedging Instrument [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	897,000	805,700
Derivative Asset, Fair Value, Gross Asset	8,230	11,689
Derivative Liability, Fair Value, Gross Liability	11,053	3,502
Not Designated as Hedging Instrument [Member] | Equity [Member]
Derivative [Line Items]
Notional Amount of Derivatives	453,435	555,361
Derivative Asset, Fair Value, Gross Asset	46,653	35,242
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument [Member] | Longevity Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	868,960	450,000
Derivative Asset, Fair Value, Gross Asset	15,003	7,727
Derivative Liability, Fair Value, Gross Liability	7	0
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	2,619	797
Not Designated as Hedging Instrument [Member] | Synthetic Guaranteed Investment Contracts [Member]
Derivative [Line Items]
Notional Amount of Derivatives	7,098,825	6,500,942
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	22,094
Derivative Liability, Fair Value, Gross Liability	76,698	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	878,114	925,887
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	192,470	159,279
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount of Derivatives	943,486	993,424
Derivative Asset, Fair Value, Gross Asset	146,265	72,081
Derivative Liability, Fair Value, Gross Liability	29,986	32,773
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	120,000	120,000
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	29,986	18,228
Designated as Hedging Instrument [Member] | Currency Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	823,486	676,972
Derivative Asset, Fair Value, Gross Asset	146,265	70,906
Derivative Liability, Fair Value, Gross Liability	0	0
Designated as Hedging Instrument [Member] | Forward Contracts [Member]
Derivative [Line Items]
Notional Amount of Derivatives	0	196,452
Derivative Asset, Fair Value, Gross Asset	0	1,175
Derivative Liability, Fair Value, Gross Liability	$ 0	$ 14,545