Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional	$ 2,484,180	$ 2,286,991
Assets	87,991	115,094
Liabilities	(91,132)	(74,378)
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	693,547	649,905
Assets	14,745	18,243
Liabilities	(65,682)	(44,806)
Derivatives Designated as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	693,547	649,905
Assets	14,745	18,243
Liabilities	(65,682)	(44,806)
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	1,790,633	1,637,086
Assets	73,246	96,851
Liabilities	(25,450)	(29,572)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Notional	246,926	246,925
Assets	16,660	23,032
Liabilities	(2,888)	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Forwards
Derivative [Line Items]
Notional	19,565	16,320
Assets	10	0
Liabilities	(177)	(376)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Credit Default Swaps
Derivative [Line Items]
Notional	756	1,594
Assets	0	0
Liabilities	(37)	(96)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	81,493	95,145
Assets	2,722	4,347
Liabilities	(7,205)	(5,600)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Options
Derivative [Line Items]
Notional	1,441,893	1,277,102
Assets	53,854	69,472
Liabilities	$ (15,143)	$ (23,500)