Allowance for Loan Losses - Summary of Roll Forward of Accruing and Nonaccruing TDRs (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Financing Receivable, Impaired [Line Items]
Beginning Balance	$ 7,770	[1]
Ending Balance	7,287
Accruing
Financing Receivable, Impaired [Line Items]
Beginning Balance	1,452
Charge-offs	0
Payments and other adjustments	(64)
New TDR designation	1,192
Release TDR designation	0
Transfer	1,835
Ending Balance	4,415
Nonaccruing
Financing Receivable, Impaired [Line Items]
Beginning Balance	2,612
Charge-offs	(92)
Payments and other adjustments	65
New TDR designation	702
Release TDR designation	0
Transfer	(1,835)
Ending Balance	1,452
Troubled Debt Restructuring
Financing Receivable, Impaired [Line Items]
Beginning Balance	4,064
Charge-offs	(92)
Payments and other adjustments	1
New TDR designation	1,894
Release TDR designation	0
Transfer	0
Ending Balance	$ 5,867

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