Accounting for Certain Loans Acquired in a Transfer - Additional Information (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Total provision charged to operating expense	$ 1,100,000	$ 710,000	$ 330,000	$ 330,000	$ 623,000	$ 455,000	$ 232,000	$ 532,000	$ 2,470,000	$ 1,842,000	$ 3,162,000
Loans Purchased With Evidence of Credit Deterioration [Member]
Business Acquisition [Line Items]
Total provision charged to operating expense									0	71,000
Allowances for loan losses									$ 71,000	$ 0