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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Loans:
Discount accretion	$ 1,982	$ 2,098	$ 559	$ 632	$ 1,489	$ 1,364	$ 2,170	$ 1,541
All other loan interest income	53,680	51,004	38,745	37,417	36,705	35,904	34,248	33,373
Total loan interest income	55,662	53,102	39,304	38,049	38,194	37,268	36,418	34,914	$ 186,117	$ 146,794	$ 141,086
Debt securities, dividends and interest bearing cash at banks	12,403	11,452	9,174	9,072	8,767	8,645	8,626	8,570
Total interest and dividend income	68,065	64,554	48,478	47,121	46,961	45,913	45,044	43,484	228,218	181,402	173,708
Interest expense	4,063	4,065	2,609	2,135	1,868	1,829	1,610	1,491	12,872	6,798	5,721
Net interest income	64,002	60,489	45,869	44,986	45,093	44,084	43,434	41,993	215,346	174,604	167,987
Provision for (benefit from reversal of provision for) loan losses	806	2,651	(638)	(236)	1,677	765	(796)	(1,557)	2,583	89	(5,970)
Net interest income after provision for loan losses	63,196	57,838	46,507	45,222	43,416	43,319	44,230	43,550	212,763	174,515	173,957
Noninterest income	12,634	12,186	12,174	12,290	12,478	12,930	12,910	11,703	49,284	50,021	44,563
Noninterest expense	45,285	47,378	37,870	38,162	38,076	37,222	35,904	35,822	168,695	147,024	145,997
Income before income taxes	30,545	22,646	20,811	19,350	17,818	19,027	21,236	19,431	93,352	77,512	72,523
Income tax expense	7,334	6,476	5,782	5,440	14,829	7,130	7,647	7,352	25,032	36,958	27,712
Net income	$ 23,211	$ 16,170	$ 15,029	$ 13,910	$ 2,989	$ 11,897	$ 13,589	$ 12,079	$ 68,320	$ 40,554	$ 44,811
Per common share:
Net income (diluted)	$ 0.76	$ 0.53	$ 0.65	$ 0.60	$ 0.13	$ 0.51	$ 0.58	$ 0.52	$ 2.54	$ 1.74	$ 1.94
Dividends	$ 0.19	$ 0.17	$ 0.17	$ 0.17	$ 0.17	$ 0.17	$ 0.17	$ 0.15