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	$ 1,259,558	$ 29,709,613
Gross Fair Value Assets	90,884	300,655
Gross Fair Value Liabilities	(12,854)	(32,849)
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	435,602	529,128
Gross Fair Value Assets	44,040	50,877
Gross Fair Value Liabilities	(1,835)	(1,385)
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	823,956	29,180,485
Gross Fair Value Assets	46,844	249,778
Gross Fair Value Liabilities	(11,019)	(31,464)
Currency Swaps | Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	435,602	529,128
Gross Fair Value Assets	44,040	50,877
Gross Fair Value Liabilities	(1,835)	(1,385)
Currency Swaps | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	56,626	122,425
Gross Fair Value Assets	7,789	17,079
Gross Fair Value Liabilities	(211)	(71)
Interest Rate Swaps | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	101,076	3,159,400
Gross Fair Value Assets	8,215	203,313
Gross Fair Value Liabilities	0	(8,605)
Foreign Currency Forwards | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	13,447	3,722
Gross Fair Value Assets	216	39
Gross Fair Value Liabilities	(20)	(15)
Credit Default Swaps | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	3,000	7,275
Gross Fair Value Assets	0	268
Gross Fair Value Liabilities	(281)	(222)
Total Return Swaps | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	0	542,294
Gross Fair Value Assets	0	411
Gross Fair Value Liabilities	0	(10,451)
Equity Options | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	649,807	25,345,369
Gross Fair Value Assets	30,624	28,668
Gross Fair Value Liabilities	$ (10,507)	$ (12,100)