Loans - Non-Accrual Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	$ 82,601	$ 150,314
Commercial Portfolio Segment [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	12,278	46,186
Commercial and industrial, Energy [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	51,802	94,302
Commercial real estate, Buildings, land and other [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	15,913	7,589
Construction Loans [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	0	0
Consumer Real Estate [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	971	2,109
Consumer and Other [Member]
Non Accrual Loans Segregated By Class Of Loans [Line Items]
Non-accrual loans	$ 1,637	$ 128