Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Notional	$ 114,937,879	$ 20,673,388
Assets	4,896,359	136,775
Liabilities	(1,659,411)	(25,277)
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	472,701	115,358
Assets	38,249	15,910
Liabilities	(2,776)	(206)
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	114,465,178	20,558,030
Assets	4,858,110	120,865
Liabilities	(1,656,635)	(25,071)
Interest Rate Swaps | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	81,872,695	1,872,750
Assets	4,439,270	84,817
Liabilities	(1,163,388)	(13,452)
Interest Rate Options | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	10,825,000	100,000
Assets	278,763	9,431
Liabilities	(135,554)	0
Interest Rate Forwards | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	498,311	0
Assets	0	0
Liabilities	(25,082)	0
Foreign Currency Forwards | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	1,491	2,752
Assets	6	23
Liabilities	0	0
Foreign Currency Swaps | Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	472,701	115,358
Assets	38,249	15,910
Liabilities	(2,776)	(206)
Foreign Currency Swaps | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	130,470	77,729
Assets	16,627	11,220
Liabilities	(635)	0
Interest Rate Futures | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	2,474,000	0
Assets	23,967	0
Liabilities	0	0
Equity Future [Member] | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	1,269,044	0
Assets	0	0
Liabilities	(5,051)	0
Total Return Swaps | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	12,784,166	217,999
Assets	69,827	320
Liabilities	(281,193)	(3,626)
Equity Options | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	4,610,001	18,286,800
Assets	29,650	15,054
Liabilities	$ (45,732)	$ (7,993)