Accounting for Certain Loans Acquired in a Transfer - Additional Information (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Total provision charged to operating expense	$ 528,000	$ 1,176,000	$ 635,000	$ 567,000	$ 1,100,000	$ 710,000	$ 330,000	$ 330,000	$ 2,906,000	$ 2,470,000	$ 1,842,000
Loans Purchased With Evidence of Credit Deterioration [Member]
Business Acquisition [Line Items]
Total provision charged to operating expense									60,000	0
Allowances for loan losses									$ 0	$ 0