Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Derivative [Line Items]
Notional	$ 26,126,550	$ 29,709,613
Assets	82,860	300,655
Liabilities	(14,847)	(32,849)
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	416,534	529,128
Assets	38,224	50,877
Liabilities	(8,079)	(1,385)
Derivatives Designated as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	416,534	529,128
Assets	38,224	50,877
Liabilities	(8,079)	(1,385)
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	25,710,016	29,180,485
Assets	44,636	249,778
Liabilities	(6,768)	(31,464)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	46,026	122,425
Assets	7,439	17,079
Liabilities	(146)	(71)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Notional	101,075	3,159,400
Assets	13,836	203,313
Liabilities	(116)	(8,605)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Forwards
Derivative [Line Items]
Notional	11,508	3,722
Assets	50	39
Liabilities	(44)	(15)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Credit Default Swaps
Derivative [Line Items]
Notional	3,000	7,275
Assets	0	268
Liabilities	(308)	(222)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Total Return Swaps
Derivative [Line Items]
Notional	0	542,294
Assets	0	411
Liabilities	0	(10,451)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Options
Derivative [Line Items]
Notional	25,548,407	25,345,369
Assets	23,311	28,668
Liabilities	$ (6,154)	$ (12,100)