Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Assets	$ 255,897	$ 251,768
Liabilities	(23,518)	(97,802)
Derivative, Notional Amount	4,390,393	4,200,893
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Assets	131,379	36,551
Liabilities	(12)	(12,521)
Derivative, Notional Amount	796,041	777,548
Derivatives Designated as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Assets	205	0
Liabilities	0	(50)
Derivative, Notional Amount	3,592	3,615
Derivatives Designated as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Assets	131,174	36,551
Liabilities	(12)	(12,471)
Derivative, Notional Amount	792,449	773,933
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Assets	124,518	215,217
Liabilities	(23,506)	(85,281)
Derivative, Notional Amount	3,594,352	3,423,345
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Assets	73,669	33,256
Liabilities	(15,394)	(4,490)
Derivative, Notional Amount	569,925	569,925
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Forwards
Derivative [Line Items]
Assets	996	0
Liabilities	(98)	(518)
Derivative, Notional Amount	28,478	21,580
Derivatives Not Qualifying as Hedge Accounting Instruments: | Credit Default Swaps
Derivative [Line Items]
Assets	0	0
Liabilities	0	0
Derivative, Notional Amount	0	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Assets	24,412	7,563
Liabilities	0	(1,892)
Derivative, Notional Amount	140,700	151,792
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Options
Derivative [Line Items]
Assets	25,441	174,398
Liabilities	(8,014)	(78,381)
Derivative, Notional Amount	$ 2,855,249	$ 2,680,048