Summary of Quarterly Results of Operations (Unaudited) - Results of Operations (Detail) - USD ($) $ / shares in Units, $ 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
Loans:
Discount accretion	$ 2,218	$ 2,360	$ 1,904	$ 1,655	$ 1,982	$ 2,098	$ 559	$ 632
All other loan interest income	54,644	54,639	53,587	52,743	53,680	51,004	38,745	37,417
Total loan interest income	56,862	56,999	55,491	54,398	55,662	53,102	39,304	38,049	$ 223,750	$ 186,117	$ 146,794
Debt securities, dividends and interest bearing cash at banks	11,056	11,890	12,689	13,059	12,403	11,452	9,174	9,072
Total interest and dividend income	67,918	68,889	68,180	67,457	68,065	64,554	48,478	47,121	272,444	228,218	181,402
Interest expense	3,722	4,201	3,865	3,587	4,063	4,065	2,609	2,135	15,375	12,872	6,798
Net interest income	64,196	64,688	64,315	63,870	64,002	60,489	45,869	44,986	257,069	215,346	174,604
(Benefit from reversal of) provision for loan losses	(298)	(329)	537	(1,600)	806	2,651	(638)	(236)	(1,690)	2,583	89
Net interest income after provision for (benefit from) loan losses	64,494	65,017	63,778	65,470	63,196	57,838	46,507	45,222	258,759	212,763	174,515
Noninterest income	14,186	14,108	13,423	11,803	12,634	12,186	12,174	12,290	53,520	49,061	49,452
Noninterest expense	46,964	46,344	46,697	45,452	45,285	47,378	37,870	38,162	185,457	168,472	146,455
Income before income taxes	31,716	32,781	30,504	31,821	30,545	22,646	20,811	19,350	126,822	93,352	77,512
Income tax expense	8,826	9,386	7,443	9,095	7,334	6,476	5,782	5,440	34,750	25,032	36,958
Net income	$ 22,890	$ 23,395	$ 23,061	$ 22,726	$ 23,211	$ 16,170	$ 15,029	$ 13,910	$ 92,072	$ 68,320	$ 40,554
Per common share:
Net income (diluted)	$ 0.75	$ 0.76	$ 0.75	$ 0.74	$ 0.76	$ 0.53	$ 0.65	$ 0.60	$ 3.00	$ 2.54	$ 1.74
Dividends	$ 0.22	$ 0.22	$ 0.19	$ 0.19	$ 0.19	$ 0.17	$ 0.17	$ 0.17	$ 0.82	$ 0.70	$ 0.66