Loans (Non-Accrual Loans, Segregated by Class of Loans) (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	$ 61,449	$ 102,303
Financing Receivable, Nonaccrual, No Allowance	21,911	24,692
Commercial Portfolio Segment [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	19,849	26,038
Financing Receivable, Nonaccrual, No Allowance	4,479	13,266
Commercial and Industrial Total Energy Loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	23,168	65,761
Financing Receivable, Nonaccrual, No Allowance	639	3,281
Paycheck Protection Program [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	0	0
Financing Receivable, Nonaccrual, No Allowance	0	0
Commercial Real Estate, Buildings, Land and Other [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	15,737	8,912
Financing Receivable, Nonaccrual, No Allowance	14,116	6,558
Construction Loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	1,684	665
Financing Receivable, Nonaccrual, No Allowance	1,684	665
Total consumer real estate loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	993	922
Financing Receivable, Nonaccrual, No Allowance	993	922
Consumer Loan [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	18	5
Financing Receivable, Nonaccrual, No Allowance	$ 0	$ 0