Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Derivative [Line Items]
Notional	$ 133,507,949	$ 20,673,388
Assets	9,039,223	136,775
Liabilities	(1,385,288)	(25,277)
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	448,210	115,358
Assets	31,621	15,910
Liabilities	(9,736)	(206)
Derivatives Designated as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	448,210	115,358
Assets	31,621	15,910
Liabilities	(9,736)	(206)
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	133,059,739	20,558,030
Assets	9,007,602	120,865
Liabilities	(1,375,552)	(25,071)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	95,352	77,729
Assets	13,979	11,220
Liabilities	(297)	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Futures
Derivative [Line Items]
Notional	2,779,000	0
Assets	0	0
Liabilities	(59,922)	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Notional	78,860,425	1,872,750
Assets	8,430,814	84,817
Liabilities	(855,591)	(13,452)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Options
Derivative [Line Items]
Notional	10,280,000	100,000
Assets	461,480	9,431
Liabilities	(72,982)	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Forwards
Derivative [Line Items]
Notional	498,411	0
Assets	39,814	0
Liabilities	0	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Forwards
Derivative [Line Items]
Notional	4,153	2,752
Assets	32	23
Liabilities	0	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Futures
Derivative [Line Items]
Notional	378,070	0
Assets	2,083	0
Liabilities	0	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Total Return Swaps
Derivative [Line Items]
Notional	16,742,863	217,999
Assets	25,346	320
Liabilities	(319,805)	(3,626)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Options
Derivative [Line Items]
Notional	23,421,465	18,286,800
Assets	34,054	15,054
Liabilities	$ (66,955)	$ (7,993)