Allowance for Loan Losses and Credit Quality Information - Schedule of Allowance for Loan Losses and Loan Balances (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Allowance for loan losses
Beginning balance	$ 40,005	$ 37,746	$ 39,751	$ 37,089	$ 37,089
Charge-offs	(3,752)	(5,744)	(8,849)	(8,376)
Recoveries	1,052	1,198	2,398	2,453
Provision (credit)	2,823	5,638	6,280	7,592
Provision for acquired loans	73	190	621	270
Ending balance	40,201	39,028	40,201	39,028	39,751
Loans individually evaluated for impairment	2,407	3,361	2,407	3,361
Loans collectively evaluated for impairment	37,252	35,393	37,252	35,393
Acquired loans evaluated for impairment			542	274
Ending balance	40,201	39,028	40,201	39,028
Loans individually evaluated for impairment	48,511	37,354	48,511	37,354
Loans collectively evaluated for impairment	4,026,672	3,551,721	4,026,672	3,551,721
Acquired nonimpaired loans	596,466	766,932	596,466	766,932	724,100
Acquired impaired loans			24,582	38,103	33,614
Ending balance	4,696,231	4,394,110	4,696,231	4,394,110	4,469,216
Owner-occupied commercial
Allowance for loan losses
Beginning balance	5,816	6,723	6,588	6,670	6,670
Charge-offs	(104)	(1,415)	(296)	(1,556)
Recoveries	12	15	120	66
Provision (credit)	(96)	1,437	(802)	1,564
Provision for acquired loans	104	185	122	201
Ending balance	5,732	6,945	5,732	6,945	6,588
Loans individually evaluated for impairment	0	0	0	0
Loans collectively evaluated for impairment	5,699	6,923	5,699	6,923
Acquired loans evaluated for impairment			33	22
Ending balance	5,732	6,945	5,732	6,945
Loans individually evaluated for impairment	3,346	2,510	3,346	2,510
Loans collectively evaluated for impairment	941,296	869,051	941,296	869,051
Acquired nonimpaired loans	144,710	175,411	144,710	175,411
Acquired impaired loans			7,401	10,673	12,122
Ending balance	1,096,753	1,057,645	1,096,753	1,057,645	1,078,162
Commercial mortgages
Allowance for loan losses
Beginning balance	7,335	8,135	8,915	6,487	6,487
Charge-offs	(1,196)	(1)	(1,702)	(79)
Recoveries	16	197	69	310
Provision (credit)	(231)	1,089	(1,602)	2,650
Provision for acquired loans	(5)	(48)	239	4
Ending balance	5,919	9,372	5,919	9,372	8,915
Loans individually evaluated for impairment	131	1,264	131	1,264
Loans collectively evaluated for impairment	5,638	7,982	5,638	7,982
Acquired loans evaluated for impairment			150	126
Ending balance	5,919	9,372	5,919	9,372
Loans individually evaluated for impairment	9,012	7,165	9,012	7,165
Loans collectively evaluated for impairment	943,699	904,328	943,699	904,328
Acquired nonimpaired loans	194,394	229,530	194,394	229,530
Acquired impaired loans			9,969	12,880	10,386
Ending balance	1,157,074	1,153,903	1,157,074	1,153,903	1,163,554
Construction
Allowance for loan losses
Beginning balance	3,432	3,308	2,838	3,521	3,521
Charge-offs	(215)	(30)	(346)	(59)
Recoveries	301	440	305	486
Provision (credit)	427	(824)	1,056	(1,104)
Provision for acquired loans	(28)	(76)	64	(26)
Ending balance	3,917	2,818	3,917	2,818	2,838
Loans individually evaluated for impairment	0	215	0	215
Loans collectively evaluated for impairment	3,881	2,549	3,881	2,549
Acquired loans evaluated for impairment			36	54
Ending balance	3,917	2,818	3,917	2,818
Loans individually evaluated for impairment	1,839	1,419	1,839	1,419
Loans collectively evaluated for impairment	271,447	182,338	271,447	182,338
Acquired nonimpaired loans	19,085	21,627	19,085	21,627
Acquired impaired loans			946	3,592	3,694
Ending balance	293,317	208,976	293,317	208,976	222,712
Complexity Risk
Allowance for loan losses
Beginning balance		0		1,010	1,010
Charge-offs		0		0
Recoveries		0		0
Provision (credit)		0		(1,010)
Provision for acquired loans		0		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	14,224	11,402	13,339	11,156	11,156
Charge-offs	(1,603)	(3,737)	(3,787)	(4,643)
Recoveries	417	223	820	557
Provision (credit)	2,128	3,714	4,597	4,551
Provision for acquired loans	(7)	117	190	98
Ending balance	15,159	11,719	15,159	11,719	13,339
Loans individually evaluated for impairment	1,220	692	1,220	692
Loans collectively evaluated for impairment	13,646	10,974	13,646	10,974
Acquired loans evaluated for impairment			293	53
Ending balance	15,159	11,719	15,159	11,719
Loans individually evaluated for impairment	12,845	4,198	12,845	4,198
Loans collectively evaluated for impairment	1,249,027	1,077,258	1,249,027	1,077,258
Acquired nonimpaired loans	120,987	175,570	120,987	175,570
Acquired impaired loans			5,235	9,691	6,183
Ending balance	1,388,094	1,266,717	1,388,094	1,266,717	1,287,731
Residential
Allowance for loan losses
Beginning balance	2,050	2,352	2,059	2,281	2,281
Charge-offs	(59)	(43)	(112)	(72)
Recoveries	11	33	141	112
Provision (credit)	(49)	(179)	(146)	(177)
Provision for acquired loans	9	12	20	31
Ending balance	1,962	2,175	1,962	2,175	2,059
Loans individually evaluated for impairment	858	989	858	989
Loans collectively evaluated for impairment	1,078	1,167	1,078	1,167
Acquired loans evaluated for impairment			26	19
Ending balance	1,962	2,175	1,962	2,175
Loans individually evaluated for impairment	14,060	13,957	14,060	13,957
Loans collectively evaluated for impairment	148,715	150,318	148,715	150,318
Acquired nonimpaired loans	77,154	103,537	77,154	103,537
Acquired impaired loans			788	899	860
Ending balance	240,717	268,711	240,717	268,711	267,028
Consumer
Allowance for loan losses
Beginning balance	7,148	5,826	6,012	5,964	5,964
Charge-offs	(575)	(518)	(2,606)	(1,967)
Recoveries	295	290	943	922
Provision (credit)	644	401	3,177	1,118
Provision for acquired loans	0	0	(14)	(38)
Ending balance	7,512	5,999	7,512	5,999	6,012
Loans individually evaluated for impairment	198	201	198	201
Loans collectively evaluated for impairment	7,310	5,798	7,310	5,798
Acquired loans evaluated for impairment			4	0
Ending balance	7,512	5,999	7,512	5,999
Loans individually evaluated for impairment	7,409	8,105	7,409	8,105
Loans collectively evaluated for impairment	472,488	368,428	472,488	368,428
Acquired nonimpaired loans	40,136	61,257	40,136	61,257
Acquired impaired loans			243	368	369
Ending balance	$ 520,276	$ 438,158	$ 520,276	$ 438,158	$ 450,029