Loans and Allowance for Credit Losses - Troubled Debt Restructured Loans and Commitments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Troubled Debt Restructurings [Line Items]
Financing Receivable, Troubled Debt Restructuring	$ 14,994	$ 20,518
Financing Receivable, Troubled Debt Restructuring, Commitment to Lend	323	1,087
Letter of Credit [Member]
Financing Receivable, Troubled Debt Restructurings [Line Items]
Financing Receivable, Troubled Debt Restructuring, Commitment to Lend	60	60
Unused lines of Credit [Member]
Financing Receivable, Troubled Debt Restructurings [Line Items]
Financing Receivable, Troubled Debt Restructuring, Commitment to Lend	263	1,027
Accrual Loans [Member] [Member]
Financing Receivable, Troubled Debt Restructurings [Line Items]
Financing Receivable, Troubled Debt Restructuring	9,120	8,757
Non Accrual Loans [Member]
Financing Receivable, Troubled Debt Restructurings [Line Items]
Financing Receivable, Troubled Debt Restructuring	$ 5,874	$ 11,761