Condensed Financial Information - Parent Company Only - Schedule of Condensed Income Statement (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statements of Income
Net gain on sales of investment securities	[1]									$ 54	$ 10	$ 48
Interest expense		$ 10,940	$ 11,655	$ 11,425	$ 10,841	$ 9,862	$ 8,832	$ 7,470	$ 6,262	44,861	32,426	19,839
Income before income taxes		18,894	21,444	20,137	19,578	22,294	18,168	4,548	15,676	80,053	60,686	61,811
Income taxes		3,384	3,782	3,669	3,499	3,922	3,204	191	2,826	14,334	10,143	17,717
Equity in undistributed income of subsidiaries:
Net income		$ 15,510	$ 17,662	$ 16,468	$ 16,079	$ 18,372	$ 14,964	$ 4,357	$ 12,850	65,719	50,543	44,094
Parent Company [Member]
Statements of Income
Dividends from Bank										29,681	22,359	26,263
Dividends from non-bank										0	0	0
Net gain on sales of investment securities										0	0	3
Other income										23,943	26,631	24,740
Total operating income										53,624	48,990	51,006
Interest expense										5,043	5,043	5,037
Operating expenses										25,032	27,155	26,405
Income before income taxes										23,549	16,792	19,564
Income taxes										(1,360)	(1,607)	(989)
Income before equity in undistributed income of subsidiaries										24,909	18,399	20,553
Equity in undistributed income of subsidiaries:
Bank										40,810	32,144	23,541
Non-banks										0	0	0
Net income										$ 65,719	$ 50,543	$ 44,094

[1] Net interest income as well as many other revenues for financial assets and liabilities including loans, leases, securities, and derivatives are excluded from the scope of FASB ASC 606 "Revenue from Contracts with Customers" (FASB ASC 606). Noninterest income streams that are out of scope of FASB ASC 606 include bank owned life insurance income, sales of investment securities and mortgage banking activities.