Credit Quality - Schedule of Nonaccrual Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans on Nonaccrual Status	$ 120,766	$ 139,671
Construction & Land Development [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans on Nonaccrual Status	16,543	16,805
Commercial Real Estate [Member] | Owner-Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans on Nonaccrual Status	17,225	13,205
Commercial Real Estate [Member] | Nonowner-Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans on Nonaccrual Status	15,623	19,814
Other Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans on Nonaccrual Status	52,249	67,536
Residential Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans on Nonaccrual Status	18,839	22,118
Consumer [Member] | Bankcard [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans on Nonaccrual Status	0	0
Consumer [Member] | Other Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans on Nonaccrual Status	$ 287	$ 193