Derivatives (Derivatives Not Designated as Hedging Instruments - Credit Contracts) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative assets - Fair value	$ 358,639	$ 107,816
Derivative liabilities - Fair value	263,734	118,305
Derivatives not designated as hedging instruments
Derivative [Line Items]
Notional amount	18,988,712	15,222,341
Derivative assets - Fair value	358,639	107,816
Derivative liabilities - Fair value	260,499	111,828
Credit contracts | Derivatives not designated as hedging instruments | Other credit derivatives
Derivative [Line Items]
Notional amount	211,343	119,320
Derivative assets - Fair value	4	1
Derivative liabilities - Fair value	122	164
Credit contracts | Derivatives not designated as hedging instruments | Other credit derivatives | RPA protection sold
Derivative [Line Items]
Notional amount	200,629	108,606
Derivative assets - Fair value	0	0
Derivative liabilities - Fair value	122	164
Credit contracts | Derivatives not designated as hedging instruments | Other credit derivatives | RPA protection purchased
Derivative [Line Items]
Notional amount	10,714	10,714
Derivative assets - Fair value	4	1
Derivative liabilities - Fair value	$ 0	$ 0