Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional	$ 123,795,359	$ 114,937,879
Assets	4,979,330	4,896,359
Liabilities	(1,660,701)	(1,659,411)
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	626,315	472,701
Assets	18,743	38,249
Liabilities	(35,244)	(2,776)
Derivatives Designated as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	626,315	472,701
Assets	18,743	38,249
Liabilities	(35,244)	(2,776)
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	123,169,044	114,465,178
Assets	4,960,587	4,858,110
Liabilities	(1,625,457)	(1,656,635)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	144,334	130,470
Assets	8,840	16,627
Liabilities	(5,588)	(635)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Futures
Derivative [Line Items]
Notional	2,034,000	2,474,000
Assets	7,982	23,967
Liabilities	0	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Notional	88,985,926	81,872,695
Assets	4,519,412	4,439,270
Liabilities	(1,062,495)	(1,163,388)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Options
Derivative [Line Items]
Notional	13,010,000	10,825,000
Assets	186,644	278,763
Liabilities	(99,906)	(135,554)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Forwards
Derivative [Line Items]
Notional	1,198,032	498,311
Assets	1,425	0
Liabilities	(5,168)	(25,082)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Forwards
Derivative [Line Items]
Notional	9,525	1,491
Assets	1	6
Liabilities	(283)	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Futures
Derivative [Line Items]
Notional	0	1,269,044
Assets	0	0
Liabilities	0	(5,051)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Total Return Swaps
Derivative [Line Items]
Notional	11,602,226	12,784,166
Assets	1,384	69,827
Liabilities	(284,030)	(281,193)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Options
Derivative [Line Items]
Notional	6,185,001	4,610,001
Assets	234,899	29,650
Liabilities	$ (167,987)	$ (45,732)