Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional	$ 3,810,870	$ 3,070,256
Assets	182,340	77,890
Liabilities	(53,828)	(26,862)
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	751,074	719,476
Assets	43,962	38,333
Liabilities	(15,484)	(16,638)
Derivatives Designated as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	751,074	719,476
Assets	43,962	38,333
Liabilities	(15,484)	(16,638)
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	3,059,796	2,350,780
Assets	138,378	39,557
Liabilities	(38,344)	(10,224)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Notional	596,925	246,925
Assets	31,580	15,665
Liabilities	(4,511)	(3,174)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Forwards
Derivative [Line Items]
Notional	27,935	23,043
Assets	262	277
Liabilities	(100)	(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	128,088	98,363
Assets	7,836	6,303
Liabilities	(1,244)	(2,109)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Options
Derivative [Line Items]
Notional	2,306,848	1,981,693
Assets	98,700	17,312
Liabilities	$ (32,489)	$ (4,912)