Deposits - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Banking and Thrift [Abstract]
Time deposits, $250,000 or more	$ 647.3	$ 550.2
Interest expense on time deposits of $100 thousand or more	1.3	1.1	$ 2.3
Deposits from certain directors, executive officers and their affiliates	$ 77.7	$ 85.6