Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Notional Amount of Derivatives	$ 11,125,976	$ 8,943,979
Derivative Asset, Fair Value, Gross Asset	260,898	101,352
Derivative Liability, Fair Value, Gross Liability	1,141,831	1,087,331
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount of Derivatives	10,132,552	8,166,174
Derivative Asset, Fair Value, Gross Asset	188,817	79,449
Derivative Liability, Fair Value, Gross Liability	1,109,058	1,082,105
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	1,144,661	1,592,943
Derivative Asset, Fair Value, Gross Asset	93,783	32,555
Derivative Liability, Fair Value, Gross Liability	3,934	21,873
Not Designated as Hedging Instrument [Member] | Interest Rate Options [Member]
Derivative [Line Items]
Notional Amount of Derivatives	240,000	240,000
Derivative Asset, Fair Value, Gross Asset	18,195	2,554
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument [Member] | Future [Member]
Derivative [Line Items]
Notional Amount of Derivatives	275,983	123,780
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	67,967	79,618
Derivative Asset, Fair Value, Gross Asset	87	0
Derivative Liability, Fair Value, Gross Liability	15,098	12,772
Not Designated as Hedging Instrument [Member] | Consumer Price Index Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	41,938	59,922
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	561	309
Not Designated as Hedging Instrument [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	805,700	682,700
Derivative Asset, Fair Value, Gross Asset	11,689	10,438
Derivative Liability, Fair Value, Gross Liability	3,502	2,156
Not Designated as Hedging Instrument [Member] | Equity [Member]
Derivative [Line Items]
Notional Amount of Derivatives	555,361	757,352
Derivative Asset, Fair Value, Gross Asset	35,242	33,902
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument [Member] | Longevity Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	450,000	0
Derivative Asset, Fair Value, Gross Asset	7,727	0
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument [Member] | Mortality Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	50,000	0
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	797	0
Not Designated as Hedging Instrument [Member] | Synthetic Guaranteed Investment Contracts [Member]
Derivative [Line Items]
Notional Amount of Derivatives	6,500,942	4,629,859
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	22,094	0
Derivative Liability, Fair Value, Gross Liability	0	176,270
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	925,887	838,670
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	159,279	30,055
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount of Derivatives	993,424	777,805
Derivative Asset, Fair Value, Gross Asset	72,081	21,903
Derivative Liability, Fair Value, Gross Liability	32,773	5,226
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	120,000	49,131
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	18,228	4,606
Designated as Hedging Instrument [Member] | Currency Swap [Member]
Derivative [Line Items]
Notional Amount of Derivatives	676,972	728,674
Derivative Asset, Fair Value, Gross Asset	70,906	21,903
Derivative Liability, Fair Value, Gross Liability	0	620
Designated as Hedging Instrument [Member] | Forward Contracts [Member]
Derivative [Line Items]
Notional Amount of Derivatives	196,452	0
Derivative Asset, Fair Value, Gross Asset	1,175	0
Derivative Liability, Fair Value, Gross Liability	$ 14,545	$ 0