Allowance for Loan Losses and Credit Quality Information - Schedule of Allowance for Loan Losses and Loan Balances (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Allowance for loan losses
Beginning balance	$ 39,826	$ 37,556	$ 39,751	$ 37,089	$ 37,089
Charge-offs	(2,378)	(1,765)	(5,097)	(2,632)
Recoveries	714	701	1,346	1,255
Provision (credit)	1,343	1,047	3,457	1,954
Provision for acquired loans	500	207	548	80
Ending balance	40,005	37,746	40,005	37,746	39,751
Loans individually evaluated for impairment	4,642	1,834	4,642	1,834
Loans collectively evaluated for impairment	34,775	35,617	34,775	35,617
Acquired loans evaluated for impairment			588	295
Ending balance	40,005	37,746	40,005	37,746
Loans individually evaluated for impairment	56,445	28,898	56,445	28,898
Loans collectively evaluated for impairment	3,888,382	3,460,885	3,888,382	3,460,885
Acquired nonimpaired loans	629,797	330,660	629,797	330,660	724,100
Acquired impaired loans			31,147	27,072	33,614
Ending balance	4,605,771	3,847,515	4,605,771	3,847,515	4,469,216
Owner-occupied commercial
Allowance for loan losses
Beginning balance	6,030	6,702	6,588	6,670	6,670
Charge-offs	0	(141)	(192)	(141)
Recoveries	33	13	108	51
Provision (credit)	(265)	133	(706)	127
Provision for acquired loans	18	16	18	16
Ending balance	5,816	6,723	5,816	6,723	6,588
Loans individually evaluated for impairment	0	0	0	0
Loans collectively evaluated for impairment	5,783	6,686	5,783	6,686
Acquired loans evaluated for impairment			33	37
Ending balance	5,816	6,723	5,816	6,723
Loans individually evaluated for impairment	3,432	838	3,432	838
Loans collectively evaluated for impairment	945,919	857,270	945,919	857,270
Acquired nonimpaired loans	148,686	70,465	148,686	70,465
Acquired impaired loans			11,839	9,757	12,122
Ending balance	1,109,876	938,330	1,109,876	938,330	1,078,162
Commercial mortgages
Allowance for loan losses
Beginning balance	8,335	6,516	8,915	6,487	6,487
Charge-offs	(402)	(61)	(506)	(78)
Recoveries	7	34	53	113
Provision (credit)	(853)	1,598	(1,371)	1,561
Provision for acquired loans	248	48	244	52
Ending balance	7,335	8,135	7,335	8,135	8,915
Loans individually evaluated for impairment	1,464	0	1,464	0
Loans collectively evaluated for impairment	5,714	8,009	5,714	8,009
Acquired loans evaluated for impairment			157	126
Ending balance	7,335	8,135	7,335	8,135
Loans individually evaluated for impairment	10,941	1,702	10,941	1,702
Loans collectively evaluated for impairment	921,069	869,771	921,069	869,771
Acquired nonimpaired loans	204,809	108,212	204,809	108,212
Acquired impaired loans			10,187	10,560	10,386
Ending balance	1,147,006	990,245	1,147,006	990,245	1,163,554
Construction
Allowance for loan losses
Beginning balance	2,961	3,609	2,838	3,521	3,521
Charge-offs	(117)	(3)	(131)	(29)
Recoveries	2	0	4	46
Provision (credit)	471	(352)	629	(280)
Provision for acquired loans	115	54	92	50
Ending balance	3,432	3,308	3,432	3,308	2,838
Loans individually evaluated for impairment	772	211	772	211
Loans collectively evaluated for impairment	2,595	3,038	2,595	3,038
Acquired loans evaluated for impairment			65	59
Ending balance	3,432	3,308	3,432	3,308
Loans individually evaluated for impairment	4,808	1,419	4,808	1,419
Loans collectively evaluated for impairment	252,637	181,440	252,637	181,440
Acquired nonimpaired loans	20,340	12,086	20,340	12,086
Acquired impaired loans			2,021	4,094	3,694
Ending balance	279,806	199,039	279,806	199,039	222,712
Complexity Risk
Allowance for loan losses
Beginning balance		1,024		1,010	1,010
Charge-offs		0		0
Recoveries		0		0
Provision (credit)		(1,024)		(1,010)
Provision for acquired loans		0
Ending balance		0		0
Loans individually evaluated for impairment		0		0
Loans collectively evaluated for impairment		0		0
Acquired loans evaluated for impairment				0
Ending balance		0		0
Loans individually evaluated for impairment		0		0
Loans collectively evaluated for impairment		0		0
Acquired nonimpaired loans		0		0
Acquired impaired loans				0
Ending balance		0		0
Commercial
Allowance for loan losses
Beginning balance	14,205	11,482	13,339	11,156	11,156
Charge-offs	(929)	(727)	(2,184)	(906)
Recoveries	319	224	403	334
Provision (credit)	520	353	2,469	837
Provision for acquired loans	109	70	197	(19)
Ending balance	14,224	11,402	14,224	11,402	13,339
Loans individually evaluated for impairment	1,342	426	1,342	426
Loans collectively evaluated for impairment	12,587	10,923	12,587	10,923
Acquired loans evaluated for impairment			295	53
Ending balance	14,224	11,402	14,224	11,402
Loans individually evaluated for impairment	14,845	2,558	14,845	2,558
Loans collectively evaluated for impairment	1,171,831	1,045,918	1,171,831	1,045,918
Acquired nonimpaired loans	130,986	58,423	130,986	58,423
Acquired impaired loans			6,002	1,711	6,183
Ending balance	1,323,664	1,108,610	1,323,664	1,108,610	1,287,731
Residential
Allowance for loan losses
Beginning balance	2,054	2,269	2,059	2,281	2,281
Charge-offs	(42)	(15)	(53)	(29)
Recoveries	10	57	130	79
Provision (credit)	17	22	(97)	2
Provision for acquired loans	11	19	11	19
Ending balance	2,050	2,352	2,050	2,352	2,059
Loans individually evaluated for impairment	869	992	869	992
Loans collectively evaluated for impairment	1,147	1,340	1,147	1,340
Acquired loans evaluated for impairment			34	20
Ending balance	2,050	2,352	2,050	2,352
Loans individually evaluated for impairment	14,023	14,416	14,023	14,416
Loans collectively evaluated for impairment	157,653	153,811	157,653	153,811
Acquired nonimpaired loans	80,074	67,484	80,074	67,484
Acquired impaired loans			852	946	860
Ending balance	252,602	236,657	252,602	236,657	267,028
Consumer
Allowance for loan losses
Beginning balance	6,241	5,954	6,012	5,964	5,964
Charge-offs	(888)	(818)	(2,031)	(1,449)
Recoveries	343	373	648	632
Provision (credit)	1,453	317	2,533	717
Provision for acquired loans	(1)	0	(14)	(38)
Ending balance	7,148	5,826	7,148	5,826	6,012
Loans individually evaluated for impairment	195	205	195	205
Loans collectively evaluated for impairment	6,949	5,621	6,949	5,621
Acquired loans evaluated for impairment			4	0
Ending balance	7,148	5,826	7,148	5,826
Loans individually evaluated for impairment	8,396	7,965	8,396	7,965
Loans collectively evaluated for impairment	439,273	352,675	439,273	352,675
Acquired nonimpaired loans	44,902	13,990	44,902	13,990
Acquired impaired loans			246	4	369
Ending balance	$ 492,817	$ 374,634	$ 492,817	$ 374,634	$ 450,029