Loans (Net (Charge-Offs)/Recoveries, Segregated by Class of Loans) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net charge-offs	$ (33,724)	$ (44,845)	$ (33,141)
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net charge-offs	(10,131)	(22,388)	(17,453)
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net charge-offs	(6,058)	(13,121)	(10,009)
Commercial Real Estate, Buildings, Land and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net charge-offs	(830)	(263)	735
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net charge-offs	24	13	11
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net charge-offs	(2,457)	(1,538)	(506)
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net charge-offs	$ (14,272)	$ (7,548)	$ (5,919)