Allowance for Credit Losses - Summary of Provision for Credit Losses (Detail) - USD ($) $ in Thousands	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
Receivables [Abstract]
Provision for loan losses									$ 8,531	$ 11,873	$ 7,846
Provision for unfunded letters of credit									1,411	(4)	255
Total provision for credit losses	$ 2,200	$ 1,640	$ 3,632	$ 2,470	$ 3,800	$ 5,840	$ 1,429	$ 800	$ 9,942	$ 11,869	$ 8,101