Derivatives (Narrative) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) contracts	Dec. 31, 2016 USD ($) contracts	Dec. 31, 2015 USD ($)
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Counterparty default losses on forward contracts	$ 0	$ 0	$ 0
Credit Derivatives [Line Items]
Commitments and contingent liabilities
Number of interest rate derivatives held | contracts	653	516
Termination value of derivatives in net liability position	$ 7,200,000	$ 34,900,000
Required collateral posting	28,200,000	50,300,000
Variation Margin Payment	20,500,000
Increase (decrease) in settlement values of the Bank's derivative assets	1,700,000	241,000
Interest Rate Swap
Credit Derivatives [Line Items]
Notional amount of credit risk derivatives	3,000,000,000	$ 2,300,000,000
Commitments To Originate Loans Held For Sale
Credit Derivatives [Line Items]
Commitments and contingent liabilities	263,999,000
Interest Rate Forward Sales Commitments
Credit Derivatives [Line Items]
Commitments and contingent liabilities	$ 447,688,000