Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional	$ 152,738,492	$ 147,248,020
Assets	4,467,949	4,847,794
Liabilities	(2,260,681)	(2,140,789)
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	838,657	768,075
Assets	33,158	33,348
Liabilities	(20,530)	(21,794)
Derivatives Designated as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	838,657	768,075
Assets	33,158	33,348
Liabilities	(20,530)	(21,794)
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	151,899,835	146,479,945
Assets	4,434,791	4,814,446
Liabilities	(2,240,151)	(2,118,995)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	216,835	231,245
Assets	10,332	11,659
Liabilities	(1,580)	(2,850)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Credit Default Swap
Derivative [Line Items]
Notional	100	0
Assets	0	0
Liabilities	(2)	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Futures
Derivative [Line Items]
Notional	2,867,600	908,100
Assets	463	4,380
Liabilities	(7,020)	(664)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Notional	86,415,325	82,172,825
Assets	3,904,603	3,344,033
Liabilities	(1,093,824)	(1,395,270)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Options
Derivative [Line Items]
Notional	18,435,000	19,255,000
Assets	159,096	139,765
Liabilities	(268,250)	(245,523)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Forwards
Derivative [Line Items]
Notional	501,438	1,713,947
Assets	29,715	56,562
Liabilities	0	(1,976)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Forwards
Derivative [Line Items]
Notional	20,058	19,467
Assets	154	287
Liabilities	(55)	(27)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Futures
Derivative [Line Items]
Notional	1,188,970	860,718
Assets	0	0
Liabilities	(6,867)	(6,629)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Total Return Swaps
Derivative [Line Items]
Notional	14,958,889	14,456,836
Assets	69,834	986,130
Liabilities	(519,910)	(53,235)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Options
Derivative [Line Items]
Notional	27,295,620	26,861,807
Assets	260,594	271,630
Liabilities	$ (342,643)	$ (412,821)