Loans - Non-Accrual Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	$ 91,578	$ 102,303
Non-accrual with no credit loss allowance	49,089	24,692
Commercial Portfolio Segment [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	21,566	26,038
Non-accrual with no credit loss allowance	6,583	13,266
Commercial and Industrial Total Energy Loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	54,041	65,761
Non-accrual with no credit loss allowance	33,324	3,281
Paycheck Protection Program [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	0	0
Non-accrual with no credit loss allowance	0	0
Commercial Real Estate Buildings Land And Other Loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	13,525	8,912
Non-accrual with no credit loss allowance	6,754	6,558
Construction Loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	680	665
Non-accrual with no credit loss allowance	680	665
Total consumer real estate loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	1,748	922
Non-accrual with no credit loss allowance	1,748	922
Consumer Loan [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	18	5
Non-accrual with no credit loss allowance	$ 0	$ 0