Loans (Details Textuals) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses	$ (265,880)	$ (278,335)	$ (274,264)
Financing Receivable, Modifications, Post-Modification Recorded Investment	9,426	9,270	41,131
Restructured Loans Subsequently Accruing	6,000
Ytd Restructured Loans Still On Nonaccrual	3,000
Loans and Leases Receivable, Impaired, Commitment to Lend	9,000
Oil and Gas Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses	$ (27,000)	$ (38,000)	$ (42,000)
Percent of Loans	4.50%