Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional	$ 124,517,190	$ 114,937,879
Assets	4,965,587	4,896,359
Liabilities	(1,497,613)	(1,659,411)
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	602,178	472,701
Assets	24,580	38,249
Liabilities	(19,610)	(2,776)
Derivatives Designated as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	602,178	472,701
Assets	24,580	38,249
Liabilities	(19,610)	(2,776)
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	123,915,012	114,465,178
Assets	4,941,007	4,858,110
Liabilities	(1,478,003)	(1,656,635)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	142,655	130,470
Assets	11,974	16,627
Liabilities	(1,497)	(635)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Futures
Derivative [Line Items]
Notional	2,159,000	2,474,000
Assets	0	23,967
Liabilities	(14,718)	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Notional	90,496,426	81,872,695
Assets	4,623,023	4,439,270
Liabilities	(1,096,749)	(1,163,388)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Options
Derivative [Line Items]
Notional	12,310,000	10,825,000
Assets	206,275	278,763
Liabilities	(108,655)	(135,554)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Forwards
Derivative [Line Items]
Notional	598,122	498,311
Assets	1,584	0
Liabilities	(1,069)	(25,082)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Forwards
Derivative [Line Items]
Notional	7,685	1,491
Assets	0	6
Liabilities	(34)	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Futures
Derivative [Line Items]
Notional	24,209	1,269,044
Assets	9	0
Liabilities	0	(5,051)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Total Return Swaps
Derivative [Line Items]
Notional	12,041,915	12,784,166
Assets	33,463	69,827
Liabilities	(202,253)	(281,193)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Options
Derivative [Line Items]
Notional	6,135,000	4,610,001
Assets	64,679	29,650
Liabilities	$ (53,028)	$ (45,732)