Accounting for Certain Loans Acquired in a Transfer - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Business Acquisition [Line Items]
Total provision charged to operating expense	$ 567,000	$ 330,000
Loans Purchased With Evidence of Credit Deterioration [Member]
Business Acquisition [Line Items]
Total provision charged to operating expense	$ 0	$ 71,000