Loans and Allowance for Credit Losses - Troubled Debt Restructured Loans and Commitments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loans and Leases Receivable Disclosure [Abstract]
Accrual status	$ 11,563	$ 13,790	$ 14,139
Nonaccrual status	11,222	11,569	12,360
Total	22,785	25,359	26,499
Letters of Credit Outstanding, Amount	60	0	0
Unused lines of credit	$ 54	$ 358	$ 3,252