Loans and Allowance for Loan Losses (Narrative) (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) contract	Jun. 30, 2017 USD ($) contract	Jun. 30, 2018 USD ($) contract	Jun. 30, 2017 USD ($) contract	Dec. 31, 2017 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment in impaired loans	$ 194,231,000	$ 232,500,000	$ 196,181,000	$ 233,800,000	$ 223,009,000
Interest income recognized	1,349,000	0	2,781,000	0	6,961,000
Interest income recognized for accruing TDRs	1,700,000	1,800,000		3,500,000
Non-accrual status	63,700,000		63,700,000		49,000,000
Recorded investment	15,878,000	3,770,000	25,365,000	15,377,000
Provision for loan losses	11,790,000	$ 10,260,000	24,566,000	$ 18,934,000
Accruing TDRs With Modifications And Renewals Completed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for loan losses			$ 6,500,000		8,700,000
Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retail loan substandard period (in days)			90 days
Loss And Charged Off
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retail loans downgraded to loss (in days)			120 days
Accruing TDRs With Modifications And Renewals Completed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accruing troubled debt restructuring	$ 125,300,000		$ 125,300,000		$ 151,300,000
Accruing TDRs With Modifications And Renewals Completed | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial-type impaired loans			$ 1,000,000
Troubled Debt Restructuring That Subsequently Defaults
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of contracts | contract	8	3	8	3
Recorded investment	$ 10,500,000	$ 292,000	$ 10,500,000	$ 292,000