Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional	$ 2,286,991	$ 1,259,558
Gross Fair Value Assets	115,094	90,884
Gross Fair Value Liabilities	(74,378)	(12,854)
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	649,905	435,602
Gross Fair Value Assets	18,243	44,040
Gross Fair Value Liabilities	(44,806)	(1,835)
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	1,637,086	823,956
Gross Fair Value Assets	96,851	46,844
Gross Fair Value Liabilities	(29,572)	(11,019)
Foreign Currency Swaps | Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	649,905	435,602
Gross Fair Value Assets	18,243	44,040
Gross Fair Value Liabilities	(44,806)	(1,835)
Foreign Currency Swaps | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	95,145	56,626
Gross Fair Value Assets	4,347	7,789
Gross Fair Value Liabilities	(5,600)	(211)
Interest Rate Swaps | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	246,925	101,076
Gross Fair Value Assets	23,032	8,215
Gross Fair Value Liabilities	0	0
Foreign Currency Forwards | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	16,320	13,447
Gross Fair Value Assets	0	216
Gross Fair Value Liabilities	(376)	(20)
Credit Default Swaps | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	1,594	3,000
Gross Fair Value Assets	0	0
Gross Fair Value Liabilities	(96)	(281)
Equity Options | Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	1,277,102	649,807
Gross Fair Value Assets	69,472	30,624
Gross Fair Value Liabilities	$ (23,500)	$ (10,507)