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, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 40,599	$ 39,751	$ 37,089
Charge-offs	(16,983)	(13,842)	(13,327)
Recoveries	2,753	3,726	3,003
Provision (credit)	13,277	10,528	12,329
Provision (credit) for acquired loans	(107)	436	657
Ending balance	39,539	40,599	39,751
Loans individually evaluated for impairment	2,031	4,658	2,895
Loans collectively evaluated for impairment	37,282	35,583	36,346
Acquired loans evaluated for impairment	226	358	510
Ending balance	39,539	40,599	39,751
Loans individually evaluated for impairment	45,007	56,315	37,071
Loans collectively evaluated for impairment	4,404,324	4,161,609	3,674,468
Acquired nonimpaired loans	429,982	565,516	724,063
Acquired impaired loans	14,944	21,654	33,614
Ending balance	4,894,257	4,805,094	4,469,216
Owner-occupied commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	5,422	6,588	6,670
Charge-offs	(417)	(296)	(1,556)
Recoveries	34	127	117
Provision (credit)	(38)	(1,098)	1,163
Provision (credit) for acquired loans	56	101	194
Ending balance	5,057	5,422	6,588
Loans individually evaluated for impairment	0	0	0
Loans collectively evaluated for impairment	4,965	5,410	6,573
Acquired loans evaluated for impairment	92	12	15
Ending balance	5,057	5,422	6,588
Loans individually evaluated for impairment	4,406	3,655	2,078
Loans collectively evaluated for impairment	938,934	933,352	899,590
Acquired nonimpaired loans	112,386	136,437	164,372
Acquired impaired loans	4,248	5,803	12,122
Ending balance	1,059,974	1,079,247	1,078,162
Commercial mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	5,891	8,915	6,487
Charge-offs	(255)	(4,612)	(422)
Recoveries	255	255	322
Provision (credit)	924	1,160	2,466
Provision (credit) for acquired loans	(9)	173	62
Ending balance	6,806	5,891	8,915
Loans individually evaluated for impairment	0	18	1,247
Loans collectively evaluated for impairment	6,727	5,779	7,482
Acquired loans evaluated for impairment	79	94	186
Ending balance	6,806	5,891	8,915
Loans individually evaluated for impairment	4,083	6,076	9,898
Loans collectively evaluated for impairment	1,005,504	983,400	921,333
Acquired nonimpaired loans	145,648	188,505	221,937
Acquired impaired loans	7,504	9,724	10,386
Ending balance	1,162,739	1,187,705	1,163,554
Construction
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	2,861	2,838	3,521
Charge-offs	(1,475)	(574)	(57)
Recoveries	3	306	484
Provision (credit)	2,341	222	(1,117)
Provision (credit) for acquired loans	(18)	69	7
Ending balance	3,712	2,861	2,838
Loans individually evaluated for impairment	444	0	217
Loans collectively evaluated for impairment	3,254	2,828	2,535
Acquired loans evaluated for impairment	14	33	86
Ending balance	3,712	2,861	2,838
Loans individually evaluated for impairment	2,781	6,022	1,419
Loans collectively evaluated for impairment	310,511	258,887	189,468
Acquired nonimpaired loans	2,525	15,759	28,131
Acquired impaired loans	749	940	3,694
Ending balance	316,566	281,608	222,712
Complexity Risk
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance		0	1,010
Charge-offs			0
Recoveries			0
Provision (credit)			(1,010)
Provision (credit) for acquired loans			0
Ending balance			0
Loans individually evaluated for impairment			0
Loans collectively evaluated for impairment			0
Acquired loans evaluated for impairment			0
Ending balance			0
Loans individually evaluated for impairment			0
Loans collectively evaluated for impairment			0
Acquired nonimpaired loans			0
Acquired impaired loans			0
Ending balance			0
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	16,732	13,339	11,156
Charge-offs	(12,130)	(5,008)	(5,052)
Recoveries	1,381	1,355	594
Provision (credit)	8,328	6,972	6,260
Provision (credit) for acquired loans	(100)	74	381
Ending balance	14,211	16,732	13,339
Loans individually evaluated for impairment	876	3,687	322
Loans collectively evaluated for impairment	13,334	12,871	12,834
Acquired loans evaluated for impairment	1	174	183
Ending balance	14,211	16,732	13,339
Loans individually evaluated for impairment	14,837	19,196	2,266
Loans collectively evaluated for impairment	1,366,151	1,324,636	1,120,193
Acquired nonimpaired loans	89,970	116,566	159,089
Acquired impaired loans	1,531	4,156	6,183
Ending balance	1,472,489	1,464,554	1,287,731
Residential
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	1,798	2,059	2,281
Charge-offs	(91)	(168)	(88)
Recoveries	154	178	254
Provision (credit)	(404)	(300)	(422)
Provision (credit) for acquired loans	(29)	29	34
Ending balance	1,428	1,798	2,059
Loans individually evaluated for impairment	543	760	911
Loans collectively evaluated for impairment	847	1,002	1,125
Acquired loans evaluated for impairment	38	36	23
Ending balance	1,428	1,798	2,059
Loans individually evaluated for impairment	11,017	13,778	13,547
Loans collectively evaluated for impairment	132,064	146,621	157,738
Acquired nonimpaired loans	57,708	72,304	94,883
Acquired impaired loans	761	784	860
Ending balance	201,550	233,487	267,028
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	7,895	6,012	5,964
Charge-offs	(2,615)	(3,184)	(6,152)
Recoveries	926	1,505	1,232
Provision (credit)	2,126	3,572	4,989
Provision (credit) for acquired loans	(7)	(10)	(21)
Ending balance	8,325	7,895	6,012
Loans individually evaluated for impairment	168	193	198
Loans collectively evaluated for impairment	8,155	7,693	5,797
Acquired loans evaluated for impairment	2	9	17
Ending balance	8,325	7,895	6,012
Loans individually evaluated for impairment	7,883	7,588	7,863
Loans collectively evaluated for impairment	651,160	514,713	386,146
Acquired nonimpaired loans	21,745	35,945	55,651
Acquired impaired loans	151	247	369
Ending balance	$ 680,939	$ 558,493	$ 450,029