Loans and Allowance for Loan Losses - Non-Accrual Loans, Loans Still Accruing and Past Due 90 Days or More and Restructured Loans (Parenthetical) (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Loans and Leases Receivable Disclosure [Line Items]
Non-accrual loans	$ 22,752	$ 17,670	$ 28,080
Troubled debt restructured loans	514	627	695
Purchased Credit Impaired Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Non-accrual loans	3,201	618	937
Doubtful [Member]
Loans and Leases Receivable Disclosure [Line Items]
Troubled debt restructured loans	$ 4,608	$ 4,629	$ 6,073