Deposits - Interest Expense on Deposits by Category, Followed on Deposits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Banking and Thrift [Abstract]
Interest-bearing demand	$ 2,211	$ 1,119	$ 666
Money market	4,690	3,343	2,466
Savings	1,017	655	289
Time deposits	4,806	3,303	3,057
Total customer interest expense	12,724	8,420	6,478
Brokered deposits	2,180	1,001	687
Total interest expense on deposits	$ 14,904	$ 9,421	$ 7,165