Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional	$ 3,070,256	$ 2,286,991
Gross Fair Value Assets	77,890	115,094
Gross Fair Value Liabilities	(26,862)	(74,378)
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	719,476	649,905
Gross Fair Value Assets	38,333	18,243
Gross Fair Value Liabilities	(16,638)	(44,806)
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	2,350,780	1,637,086
Gross Fair Value Assets	39,557	96,851
Gross Fair Value Liabilities	(10,224)	(29,572)
Foreign Currency Swaps | Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	719,476	649,905
Gross Fair Value Assets	38,333	18,243
Gross Fair Value Liabilities	(16,638)	(44,806)
Foreign Currency Swaps | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	98,363	95,145
Gross Fair Value Assets	6,303	4,347
Gross Fair Value Liabilities	(2,109)	(5,600)
Interest Rate Swaps | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	246,925	246,925
Gross Fair Value Assets	15,665	23,032
Gross Fair Value Liabilities	(3,174)	0
Foreign Currency Forwards | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	23,043	16,320
Gross Fair Value Assets	277	0
Gross Fair Value Liabilities	(20)	(376)
Credit Default Swaps | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	756	1,594
Gross Fair Value Assets	0	0
Gross Fair Value Liabilities	(9)	(96)
Equity Options | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	1,981,693	1,277,102
Gross Fair Value Assets	17,312	69,472
Gross Fair Value Liabilities	$ (4,912)	$ (23,500)