Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative, Notional Amount	$ 150,758,714	$ 147,248,020
Assets	10,400,440	4,847,794
Liabilities	(3,825,724)	(2,140,789)
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Derivative, Notional Amount	1,011,740	768,075
Assets	66,182	33,348
Liabilities	(6,817)	(21,794)
Derivatives Designated as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Derivative, Notional Amount	1,011,740	768,075
Assets	66,182	33,348
Liabilities	(6,817)	(21,794)
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Derivative, Notional Amount	149,746,974	146,479,945
Assets	10,334,258	4,814,446
Liabilities	(3,818,907)	(2,118,995)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Derivative, Notional Amount	204,104	231,245
Assets	20,804	11,659
Liabilities	0	(2,850)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Credit Default Swap
Derivative [Line Items]
Derivative, Notional Amount	0	0
Assets	0	0
Liabilities	0	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Futures
Derivative [Line Items]
Derivative, Notional Amount	4,315,800	908,100
Assets	408	4,380
Liabilities	(1,585)	(664)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Derivative, Notional Amount	88,656,425	82,172,825
Assets	9,135,289	3,344,033
Liabilities	(3,093,358)	(1,395,270)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Options
Derivative [Line Items]
Derivative, Notional Amount	14,730,000	19,255,000
Assets	820,638	139,765
Liabilities	(193,893)	(245,523)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Forwards
Derivative [Line Items]
Derivative, Notional Amount	198,834	1,713,947
Assets	33,269	56,562
Liabilities	0	(1,976)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Forwards
Derivative [Line Items]
Derivative, Notional Amount	16,331	19,467
Assets	192	287
Liabilities	(65)	(27)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Futures
Derivative [Line Items]
Derivative, Notional Amount	1,288,315	860,718
Assets	209	0
Liabilities	(4,923)	(6,629)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Total Return Swaps
Derivative [Line Items]
Derivative, Notional Amount	15,960,466	14,456,836
Assets	114,009	986,130
Liabilities	(136,596)	(53,235)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Options
Derivative [Line Items]
Derivative, Notional Amount	24,376,699	26,861,807
Assets	209,440	271,630
Liabilities	$ (388,487)	$ (412,821)