Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional	$ 147,148,666	$ 153,711,188
Assets	4,115,765	5,070,664
Liabilities	(2,615,112)	(1,941,403)
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	704,065	677,257
Assets	16,975	13,348
Liabilities	(40,722)	(47,209)
Derivatives Designated as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	704,065	677,257
Assets	16,975	13,348
Liabilities	(40,722)	(47,209)
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	146,444,601	153,033,931
Assets	4,098,790	5,057,316
Liabilities	(2,574,390)	(1,894,194)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	150,036	151,400
Assets	8,485	7,779
Liabilities	(6,279)	(7,488)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Futures
Derivative [Line Items]
Notional	1,182,800	1,964,000
Assets	0	8,296
Liabilities	(685)	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Notional	80,473,825	87,939,425
Assets	3,383,095	4,374,658
Liabilities	(1,844,343)	(1,065,549)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Options
Derivative [Line Items]
Notional	17,925,000	15,775,000
Assets	162,082	175,156
Liabilities	(224,646)	(160,181)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Forwards
Derivative [Line Items]
Notional	1,201,802	975,929
Assets	12,961	19,870
Liabilities	(903)	(2)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Forwards
Derivative [Line Items]
Notional	12,882	12,455
Assets	638	1
Liabilities	0	(319)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Futures
Derivative [Line Items]
Notional	9,607	672,055
Assets	82	2,442
Liabilities	0	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Total Return Swaps
Derivative [Line Items]
Notional	13,328,751	13,841,333
Assets	181,539	8,517
Liabilities	(126,354)	(341,700)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Options
Derivative [Line Items]
Notional	32,159,898	31,702,334
Assets	349,908	460,597
Liabilities	$ (371,180)	$ (318,955)