Investment Securities - Information Related to Sales of Securities Available-for-Sale (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Securities available-for-sale:
Proceeds from sales	$ 7,069	$ 77,290	$ 77,308
Gross realized gains on sales	74	600	1,295
Gross realized losses on sales	26	82	30
Tax expense related to net realized gains on sales	$ 17	$ 181	$ 443