Loans and Allowance for Credit Losses - Troubled Debt Restructured Loans and Commitments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Troubled Debt Restructurings [Line Items]
Financing Receivable, Modifications, Recorded Investment	$ 28,940	$ 22,785
Loans and Leases Receivable, Impaired, Commitment to Lend	1,838	114
Letter of Credit [Member]
Financing Receivable, Troubled Debt Restructurings [Line Items]
Loans and Leases Receivable, Impaired, Commitment to Lend	60	60
Unused lines of Credit [Member]
Financing Receivable, Troubled Debt Restructurings [Line Items]
Loans and Leases Receivable, Impaired, Commitment to Lend	1,778	54
Accrual Loans [Member] [Member]
Financing Receivable, Troubled Debt Restructurings [Line Items]
Financing Receivable, Modifications, Recorded Investment	18,707	11,563
Non Accrual Loans [Member]
Financing Receivable, Troubled Debt Restructurings [Line Items]
Financing Receivable, Modifications, Recorded Investment	$ 10,233	$ 11,222