Fair Value Disclosures - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
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	22,752,000	28,080,000	17,670,000
Impaired loans valuation reserves	4,365,000	5,072,000	3,996,000
Impaired loans net fair value	18,387,000
Recorded Investment With No Allowance	5,682,000	7,725,000	3,009,000
Other real estate owned, total	$ 1,179,000	$ 1,207,000	$ 1,347,000
Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt discounts, percentage	5.00%
Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt discounts, percentage	25.00%