Fair Value Disclosures - Additional Information (Detail)	9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets transfer between Level 1 and Level 2	$ 0	$ 0	$ 0
Assets transfer between Level 2 and Level 3	0	0	0
Impaired loans carrying value	25,587,000	18,750,000	17,670,000
Impaired loans valuation reserves	4,988,000	4,177,000	3,996,000
Impaired loans net fair value	20,599,000
Recorded Investment With No Allowance	7,732,000	4,621,000	3,009,000
Other real estate owned, total	$ 593,000	$ 2,176,000	$ 1,347,000
Minimum [Member] | Measurement Input, Discount Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt discounts, percentage	5
Maximum [Member] | Measurement Input, Discount Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt discounts, percentage	25