Summary of Quarterly Results of Operations (Unaudited) - Results of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017
Loans:
Discount accretion PCI - cash basis	$ 246	$ 516	$ 398	$ 386	$ 112
Discount accretion PCI - other	60	445	407	797	631
Discount accretion PNCI	326	528	559	987	798
All other loan interest income	37,417	36,705	35,904	34,248	33,373
Total loan interest income	38,049	38,194	37,268	36,418	34,914
Debt securities, dividends and interest bearing cash at banks (not FTE)	9,072	8,767	8,645	8,626	8,570
Total interest and dividend income	47,121	46,961	45,913	45,044	43,484
Interest expense	2,135	1,868	1,829	1,610	1,491
Net interest income	44,986	45,093	44,084	43,434	41,993
Provision for (benefit from reversal of provision for) loan losses	(236)	1,677	765	(796)	(1,557)
Net interest income after provision for loan losses	45,222	43,416	43,319	44,230	43,550
Noninterest income	12,290	12,478	12,930	12,910	11,703
Noninterest expense	38,162	38,076	37,222	35,904	35,822
Income before income taxes	19,350	17,818	19,027	21,236	19,431
Income tax expense	5,440	14,829	7,130	7,647	7,352
Net income	$ 13,910	$ 2,989	$ 11,897	$ 13,589	$ 12,079
Per common share:
Net income (diluted)	$ 0.60	$ 0.13	$ 0.51	$ 0.58	$ 0.52
Dividends	$ 0.17	$ 0.17	$ 0.17	$ 0.17	$ 0.15