Loans - Net (Charge-Offs)/Recoveries, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	$ (6,235)	$ (4,986)	$ (26,100)	$ (28,820)
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	(4,565)	(3,079)	(12,155)	(8,177)
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	451	(865)	(10,010)	(18,623)
Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	266	259	768	801
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	2	9	8	18
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	(629)	(195)	(422)	(22)
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
(Net charge-offs)/Recoveries	$ (1,760)	$ (1,115)	$ (4,289)	$ (2,817)