Summary of Quarterly Results of Operations (Unaudited) - Results of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						6 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017
Loans:
Discount accretion PCI - cash basis	$ 180	$ 246	$ 516	$ 398	$ 386	$ 112
Discount accretion PCI - other	95	60	445	407	797	631
Discount accretion PNCI	284	326	528	559	987	798
All other loan interest income	38,745	37,417	36,705	35,904	34,248	33,373
Total loan interest income	39,304	38,049	38,194	37,268	36,418	34,914	$ 77,353	$ 71,332
Debt securities, dividends and interest bearing cash at banks (not FTE)	9,174	9,072	8,767	8,645	8,626	8,570
Total interest and dividend income	48,478	47,121	46,961	45,913	45,044	43,484	95,599	88,528
Interest expense	2,609	2,135	1,868	1,829	1,610	1,491	4,744	3,101
Net interest income	45,869	44,986	45,093	44,084	43,434	41,993	90,855	85,427
Provision for (benefit from reversal of provision for) loan losses	(638)	(236)	1,677	765	(796)	(1,557)	(874)	(2,353)
Net interest income after provision for loan losses	46,507	45,222	43,416	43,319	44,230	43,550	91,729	87,780
Noninterest income	12,174	12,290	12,478	12,930	12,910	11,703	24,464	24,613
Noninterest expense	37,870	38,162	38,076	37,222	35,904	35,822	76,032	71,726
Income before income taxes	20,811	19,350	17,818	19,027	21,236	19,431	40,161	40,667
Income tax expense	5,782	5,440	14,829	7,130	7,647	7,352	11,222	14,999
Net income	$ 15,029	$ 13,910	$ 2,989	$ 11,897	$ 13,589	$ 12,079	$ 28,939	$ 25,668
Per common share:
Net income (diluted)	$ 0.65	$ 0.60	$ 0.13	$ 0.51	$ 0.58	$ 0.52	$ 1.24	$ 1.10
Dividends	$ 0.17	$ 0.17	$ 0.17	$ 0.17	$ 0.17	$ 0.15