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, 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
Loans:
Discount accretion	$ 1,960	$ 1,876	$ 2,587	$ 1,748	$ 2,218	$ 2,360	$ 1,904	$ 1,655
All other loan interest income	59,055	56,163	55,822	54,510	54,644	54,639	53,587	52,743
Total loan interest income	61,015	58,039	58,409	56,258	56,862	56,999	55,491	54,398	$ 233,721	$ 223,750	$ 186,117
Debt securities, dividends and interest bearing cash at banks	7,066	7,399	8,739	10,259	11,056	11,890	12,689	13,059
Total interest and dividend income	68,081	65,438	67,148	66,517	67,918	68,889	68,180	67,457	267,184	272,444	228,218
Interest expense	1,659	1,984	2,489	3,325	3,722	4,201	3,865	3,587	9,457	15,375	12,872
Net interest income	66,422	63,454	64,659	63,192	64,196	64,688	64,315	63,870	257,727	257,069	215,346
Provision for credit losses	4,850	7,649	22,244	8,070	(298)	(329)	537	(1,600)
Net interest income after provision for (benefit from) credit losses	61,572	55,805	42,415	55,122	64,494	65,017	63,778	65,470	214,914	258,759	212,763
Noninterest income	16,580	15,137	11,657	11,820	14,186	14,108	13,423	11,803	55,194	53,520	49,061
Total noninterest expense	45,745	46,714	45,550	44,749	46,964	46,344	46,697	45,452	182,758	185,457	168,472
Income before income taxes	32,407	24,228	8,522	22,193	31,716	32,781	30,504	31,821	87,350	126,822	93,352
Income tax expense	8,750	6,622	1,092	6,072	8,826	9,386	7,443	9,095	22,536	34,750	25,032
Net income	$ 23,657	$ 17,606	$ 7,430	$ 16,121	$ 22,890	$ 23,395	$ 23,061	$ 22,726	$ 64,814	$ 92,072	$ 68,320
Per common share:
Net income (diluted) (in USD per share)	$ 0.79	$ 0.59	$ 0.25	$ 0.53	$ 0.75	$ 0.76	$ 0.75	$ 0.74	$ 2.16	$ 3.00	$ 2.54
Dividends paid (in USD per share)	$ 0.22	$ 0.22	$ 0.22	$ 0.22	$ 0.22	$ 0.22	$ 0.19	$ 0.19	$ 0.88	$ 0.82	$ 0.70