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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loans:
Discount accretion PCI - cash basis	$ 516	$ 398	$ 386	$ 112	$ 483	$ 777	$ 426	$ 269
Discount accretion PCI - other	445	407	797	631	658	569	415	(45)
Discount accretion PNCI	528	559	987	798	637	883	1,459	868
All other loan interest income	36,705	35,904	34,248	33,373	34,463	33,540	32,038	33,646
Total loan interest income	38,194	37,268	36,418	34,914	36,241	35,769	34,338	34,738	$ 146,794	$ 141,086	$ 131,836
Debt securities, dividends and interest bearing cash at Banks (not FTE)	8,767	8,645	8,626	8,570	8,374	7,940	8,252	8,056
Total interest and dividend income	46,961	45,913	45,044	43,484	44,615	43,709	42,590	42,794	181,402	173,708	161,414
Interest expense	1,868	1,829	1,610	1,491	1,460	1,439	1,430	1,392	6,798	5,721	5,416
Net interest income	45,093	44,084	43,434	41,993	43,155	42,270	41,160	41,402	174,604	167,987	155,998
(Benefit from reversal of) provision for loan losses	1,677	765	(796)	(1,557)	(1,433)	(3,973)	(773)	209	89	(5,970)	(2,210)
Net interest income after provision for loan losses	43,416	43,319	44,230	43,550	44,588	46,243	41,933	41,193	174,515	173,957	158,208
Noninterest income	12,478	12,930	12,910	11,703	12,462	11,066	11,245	9,790	50,021	44,563	45,347
Noninterest expense	38,076	37,222	35,904	35,822	36,563	37,416	38,267	33,751	147,024	145,997	130,841
Income before income taxes	17,818	19,027	21,236	19,431	20,487	19,893	14,911	17,232	77,512	72,523	72,714
Income tax expense	14,829	7,130	7,647	7,352	7,954	7,694	5,506	6,558	36,958	27,712	28,896
Net income	$ 2,989	$ 11,897	$ 13,589	$ 12,079	$ 12,533	$ 12,199	$ 9,405	$ 10,674	$ 40,554	$ 44,811	$ 43,818
Per common share:
Net income (diluted)	$ 0.13	$ 0.51	$ 0.58	$ 0.52	$ 0.54	$ 0.53	$ 0.41	$ 0.46	$ 1.74	$ 1.94	$ 1.91
Dividends	$ 0.17	$ 0.17	$ 0.17	$ 0.15	$ 0.15	$ 0.15	$ 0.15	$ 0.15	$ 0.66	$ 0.60	$ 0.52