Loans (Non-Accrual Loans, Segregated by Class of Loans) (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	$ 102,303	$ 73,739
Commercial Portfolio Segment [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	26,038	9,239
Commercial and Industrial Total Energy Loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	65,761	46,932
Commercial Real Estate, Buildings, Land and Other [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	8,912	15,268
Construction Loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	665	0
Total consumer real estate loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	922	892
Consumer Loan [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	$ 5	$ 1,408