Condensed Financial Information - Parent Company Only - Schedule of Condensed Income Statement (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
Statements of Income
Net gain on sales of investment securities									$ 48	$ 518	$ 1,265
Interest expense	$ 5,711	$ 5,285	$ 4,730	$ 4,113	$ 3,884	$ 3,836	$ 2,451	$ 2,211	19,839	12,382	8,065
Income before income taxes	15,426	15,612	15,995	14,778	7,486	(1,475)	7,286	10,089	61,811	23,386	37,026
Income taxes	5,162	4,416	4,217	3,922	568	(1,533)	2,046	2,800	17,717	3,881	9,758
Equity in undistributed income (loss) of subsidiaries:
Net income	$ 10,264	$ 11,196	$ 11,778	$ 10,856	$ 6,918	$ 58	$ 5,240	$ 7,289	44,094	19,505	27,268
Parent Company [Member]
Statements of Income
Dividends from Bank									26,263	94,042	26,523
Dividends from non-bank									0	0	0
Net gain on sales of investment securities									3	23	285
Other income									24,740	18,663	18,428
Total operating income									51,006	112,728	45,236
Interest expense									5,037	4,158	2,023
Operating expenses									26,405	25,843	19,810
Income before income taxes									19,564	82,727	23,403
Income taxes									(989)	(3,834)	(728)
Income before equity in undistributed income (loss) of subsidiaries									20,553	86,561	24,131
Equity in undistributed income (loss) of subsidiaries:
Bank									23,541	(67,056)	3,137
Non-banks									0	0	0
Net income									$ 44,094	$ 19,505	$ 27,268