Noninterest Income and Expense - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Mortgage loan servicing fees, net of change in fair value of mortgage loan servicing rights	$ 1,358,000	$ (119,000)	$ 1,463,000