Condensed Financial Information - Parent Company Only - Schedule of Condensed Income Statement (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statements of Income
Interest expense	$ 6,813	$ 6,758	$ 6,462	$ 9,551	$ 10,940	$ 11,655	$ 11,425	$ 10,841	$ 29,584	$ 44,861	$ 32,426
Income before income taxes	31,647	23,197	1,821	232	18,894	21,444	20,137	19,578	56,897	80,053	60,686
Income tax expense	5,773	5,078	(264)	(606)	3,384	3,782	3,669	3,499	9,981	14,334	10,143
Equity in undistributed income of subsidiaries:
Net income	$ 25,874	$ 18,119	$ 2,085	$ 838	$ 15,510	$ 17,662	$ 16,468	$ 16,079	46,916	65,719	50,543
Parent Company [Member]
Statements of Income
Dividends from Bank									9,746	29,681	22,359
Dividends from non-bank									0	0	0
Other income									21,803	23,943	26,631
Total operating income									31,549	53,624	48,990
Interest expense									6,762	5,043	5,043
Operating expenses									22,852	25,032	27,155
Income before income taxes									1,935	23,549	16,792
Income tax expense									(1,555)	(1,360)	(1,607)
Income before equity in undistributed income of subsidiaries									3,490	24,909	18,399
Equity in undistributed income of subsidiaries:
Bank									43,426	40,810	32,144
Non-banks									0	0	0
Net income									$ 46,916	$ 65,719	$ 50,543