Derivatives (Derivatives Not Designated as Hedging Instruments - Credit Contracts) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative assets - Fair value	$ 107,816	$ 67,077
Derivative liabilities - Fair Value	118,305	75,838
Derivative instruments not designated as hedging instruments
Derivative [Line Items]
Notional amount	15,222,341	10,153,108
Derivative assets - Fair value	107,816	67,077
Derivative liabilities - Fair Value	111,828	69,068
Derivative instruments not designated as hedging instruments | Credit contracts
Derivative [Line Items]
Notional amount	119,320	49,033
Derivative assets - Fair value	1	1
Derivative liabilities - Fair Value	164	8
Credit contracts | Derivative instruments not designated as hedging instruments | Credit contracts
Derivative [Line Items]
Notional amount	119,320	49,033
Derivative assets - Fair value	1	1
Derivative liabilities - Fair Value	164	8
Credit contracts | Derivative instruments not designated as hedging instruments | Credit contracts | RPAs - protection sold
Derivative [Line Items]
Notional amount	108,606	35,208
Derivative assets - Fair value	0	0
Derivative liabilities - Fair Value	164	8
Credit contracts | Derivative instruments not designated as hedging instruments | Credit contracts | RPAs - protection purchased
Derivative [Line Items]
Notional amount	10,714	13,825
Derivative assets - Fair value	1	1
Derivative liabilities - Fair Value	$ 0	$ 0