Allowance for Loan Losses and Credit Quality Information - Schedule of Allowance for Loan Losses and Loan Balances (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 39,751	$ 37,089	$ 39,426
Charge-offs	(13,842)	(13,327)	(12,095)
Recoveries	3,726	3,003	1,968
Provision (credit) for loan losses	10,528	12,329	7,440
Provision for acquired loans	436	657	350
Ending balance	40,599	39,751	37,089
Loans individually evaluated for impairment	4,658	2,895	2,492
Loans collectively evaluated for impairment	35,583	36,346	34,465
Acquired loans evaluated for impairment	358	510	132
Ending balance	40,599	39,751	37,089
Loans individually evaluated for impairment	56,315	37,071	34,812
Loans collectively evaluated for impairment	4,161,609	3,674,468	3,335,992
Acquired nonimpaired loans	565,516	724,063	371,148
Acquired impaired loans	21,654	33,614	32,687
Ending balance	4,805,094	4,469,216	3,774,639
Owner-occupied commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	6,588	6,670	6,643
Charge-offs	(296)	(1,556)	(738)
Recoveries	127	117	77
Provision (credit) for loan losses	(1,098)	1,163	665
Provision for acquired loans	101	194	23
Ending balance	5,422	6,588	6,670
Loans individually evaluated for impairment	0	0	0
Loans collectively evaluated for impairment	5,410	6,573	6,648
Acquired loans evaluated for impairment	12	15	22
Ending balance	5,422	6,588	6,670
Loans individually evaluated for impairment	3,655	2,078	1,090
Loans collectively evaluated for impairment	933,352	899,590	820,911
Acquired nonimpaired loans	136,437	164,372	53,954
Acquired impaired loans	5,803	12,122	4,688
Ending balance	1,079,247	1,078,162	880,643
Commercial mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	8,915	6,487	7,266
Charge-offs	(4,612)	(422)	(1,135)
Recoveries	255	322	222
Provision (credit) for loan losses	1,160	2,466	(67)
Provision for acquired loans	173	62	201
Ending balance	5,891	8,915	6,487
Loans individually evaluated for impairment	18	1,247	0
Loans collectively evaluated for impairment	5,779	7,482	6,384
Acquired loans evaluated for impairment	94	186	103
Ending balance	5,891	8,915	6,487
Loans individually evaluated for impairment	6,076	9,898	3,411
Loans collectively evaluated for impairment	983,400	921,333	869,359
Acquired nonimpaired loans	188,505	221,937	83,415
Acquired impaired loans	9,724	10,386	10,513
Ending balance	1,187,705	1,163,554	966,698
Construction
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	2,838	3,521	2,596
Charge-offs	(574)	(57)	(146)
Recoveries	306	484	185
Provision (credit) for loan losses	222	(1,117)	852
Provision for acquired loans	69	7	34
Ending balance	2,861	2,838	3,521
Loans individually evaluated for impairment	0	217	211
Loans collectively evaluated for impairment	2,828	2,535	3,310
Acquired loans evaluated for impairment	33	86	0
Ending balance	2,861	2,838	3,521
Loans individually evaluated for impairment	6,022	1,419	1,419
Loans collectively evaluated for impairment	258,887	189,468	213,801
Acquired nonimpaired loans	15,759	28,131	27,009
Acquired impaired loans	940	3,694	3,544
Ending balance	281,608	222,712	245,773
Complexity Risk
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	0	1,010	1,520
Charge-offs		0	0
Recoveries		0	0
Provision (credit) for loan losses		(1,010)	(510)
Provision for acquired loans		0	0
Ending balance		0	1,010
Loans individually evaluated for impairment		0	0
Loans collectively evaluated for impairment		0	1,010
Acquired loans evaluated for impairment		0	0
Ending balance		0	1,010
Loans individually evaluated for impairment		0	0
Loans collectively evaluated for impairment		0	0
Acquired nonimpaired loans		0	0
Acquired impaired loans		0	0
Ending balance		0	0
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	13,339	11,156	12,837
Charge-offs	(5,008)	(5,052)	(6,303)
Recoveries	1,355	594	301
Provision (credit) for loan losses	6,972	6,260	4,241
Provision for acquired loans	74	381	80
Ending balance	16,732	13,339	11,156
Loans individually evaluated for impairment	3,687	322	1,164
Loans collectively evaluated for impairment	12,871	12,834	9,988
Acquired loans evaluated for impairment	174	183	4
Ending balance	16,732	13,339	11,156
Loans individually evaluated for impairment	19,196	2,266	5,680
Loans collectively evaluated for impairment	1,324,636	1,120,193	930,346
Acquired nonimpaired loans	116,566	159,089	112,586
Acquired impaired loans	4,156	6,183	12,985
Ending balance	1,464,554	1,287,731	1,061,597
Residential
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	2,059	2,281	2,523
Charge-offs	(168)	(88)	(548)
Recoveries	178	254	226
Provision (credit) for loan losses	(300)	(422)	76
Provision for acquired loans	29	34	4
Ending balance	1,798	2,059	2,281
Loans individually evaluated for impairment	760	911	918
Loans collectively evaluated for impairment	1,002	1,125	1,360
Acquired loans evaluated for impairment	36	23	3
Ending balance	1,798	2,059	2,281
Loans individually evaluated for impairment	13,778	13,547	15,548
Loans collectively evaluated for impairment	146,621	157,738	166,252
Acquired nonimpaired loans	72,304	94,883	76,929
Acquired impaired loans	784	860	950
Ending balance	233,487	267,028	259,679
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	6,012	5,964	6,041
Charge-offs	(3,184)	(6,152)	(3,225)
Recoveries	1,505	1,232	957
Provision (credit) for loan losses	3,572	4,989	2,183
Provision for acquired loans	(10)	(21)	8
Ending balance	7,895	6,012	5,964
Loans individually evaluated for impairment	193	198	199
Loans collectively evaluated for impairment	7,693	5,797	5,765
Acquired loans evaluated for impairment	9	17	0
Ending balance	7,895	6,012	5,964
Loans individually evaluated for impairment	7,588	7,863	7,664
Loans collectively evaluated for impairment	514,713	386,146	335,323
Acquired nonimpaired loans	35,945	55,651	17,255
Acquired impaired loans	247	369	7
Ending balance	$ 558,493	$ 450,029	$ 360,249