Allowance for Loan Losses - Summary of Roll Forward of Accruing and Nonaccruing TDRs (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Financing Receivable, Impaired [Line Items]
Beginning Balance	$ 7,770	[1]
Ending Balance	7,113
Accruing
Financing Receivable, Impaired [Line Items]
Beginning Balance	1,452
Charge-offs	0
Payments and other adjustments	(41)
New TDR designation	557
Release TDR designation	0
Transfer	1,835
Ending Balance	3,803
Nonaccruing
Financing Receivable, Impaired [Line Items]
Beginning Balance	2,612
Charge-offs	(92)
Payments and other adjustments	7
New TDR designation	30
Release TDR designation	0
Transfer	(1,729)
Ending Balance	828
Troubled Debt Restructuring
Financing Receivable, Impaired [Line Items]
Beginning Balance	4,064
Charge-offs	(92)
Payments and other adjustments	(34)
New TDR designation	587
Release TDR designation	0
Transfer	106
Ending Balance	$ 4,631

[1] Derived from audited December 31, 2017 Consolidated Financial Statements.