Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional	$ 3,245,456	$ 3,070,256
Assets	131,368	77,890
Liabilities	(39,763)	(26,862)
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	738,998	719,476
Assets	33,171	38,333
Liabilities	(19,619)	(16,638)
Derivatives Designated as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	738,998	719,476
Assets	33,171	38,333
Liabilities	(19,619)	(16,638)
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	2,506,458	2,350,780
Assets	98,197	39,557
Liabilities	(20,144)	(10,224)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Notional	246,925	246,925
Assets	20,945	15,665
Liabilities	0	(3,174)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Forwards
Derivative [Line Items]
Notional	23,318	23,043
Assets	331	277
Liabilities	(38)	(20)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Credit Default Swaps
Derivative [Line Items]
Notional	0	756
Assets	0	0
Liabilities	0	(9)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	113,062	98,363
Assets	5,987	6,303
Liabilities	(1,035)	(2,109)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Options
Derivative [Line Items]
Notional	2,123,153	1,981,693
Assets	70,934	17,312
Liabilities	$ (19,071)	$ (4,912)