ALLOWANCE FOR LOAN LOSSES AND CREDIT QUALITY INFORMATION - Schedule of Allowance for Loan Losses and Loan Balances (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Allowance for loan losses
Beginning balance	$ 40,599	$ 39,751	$ 39,751
Charge-offs	(3,880)	(2,719)
Recoveries	441	632
Provision (credit)	3,655	2,114
Provision for acquired loans	(5)	48
Ending balance	40,810	39,826	40,599
Loans individually evaluated for impairment	3,461	4,836
Loans collectively evaluated for impairment	37,023	34,526
Acquired loans evaluated for impairment	326	464
Ending balance	40,810	39,826
Loans individually evaluated for impairment	54,309	56,997
Loans collectively evaluated for impairment	4,225,123	3,813,777
Acquired nonimpaired loans	534,841	675,607	565,500
Acquired impaired loans	20,154	31,247	21,654
Ending balance	4,834,427	4,577,628	4,805,094
Owner-occupied commercial
Allowance for loan losses
Beginning balance	5,422	6,588	6,588
Charge-offs	(10)	(192)
Recoveries	5	75
Provision (credit)	(58)	(441)
Provision for acquired loans	0	0
Ending balance	5,359	6,030	5,422
Loans individually evaluated for impairment	0	0
Loans collectively evaluated for impairment	5,347	6,015
Acquired loans evaluated for impairment	12	15
Ending balance	5,359	6,030
Loans individually evaluated for impairment	4,342	4,020
Loans collectively evaluated for impairment	938,166	919,508
Acquired nonimpaired loans	133,007	156,724
Acquired impaired loans	5,147	12,040	5,803
Ending balance	1,080,662	1,092,292	1,079,247
Commercial mortgages
Allowance for loan losses
Beginning balance	5,891	8,915	8,915
Charge-offs	(48)	(104)
Recoveries	134	46
Provision (credit)	617	(518)
Provision for acquired loans	23	(4)
Ending balance	6,617	8,335	5,891
Loans individually evaluated for impairment	0	1,395
Loans collectively evaluated for impairment	6,528	6,763
Acquired loans evaluated for impairment	89	177
Ending balance	6,617	8,335
Loans individually evaluated for impairment	5,946	9,771
Loans collectively evaluated for impairment	970,750	928,925
Acquired nonimpaired loans	178,518	216,410
Acquired impaired loans	9,210	10,450	9,724
Ending balance	1,164,424	1,165,556	1,187,705
Construction
Allowance for loan losses
Beginning balance	2,861	2,838	2,838
Charge-offs	0	(14)
Recoveries	1	2
Provision (credit)	27	158
Provision for acquired loans	(25)	(23)
Ending balance	2,864	2,961	2,861
Loans individually evaluated for impairment	0	500
Loans collectively evaluated for impairment	2,857	2,397
Acquired loans evaluated for impairment	7	64
Ending balance	2,864	2,961
Loans individually evaluated for impairment	6,490	3,130
Loans collectively evaluated for impairment	267,293	225,718
Acquired nonimpaired loans	15,259	24,540
Acquired impaired loans	901	2,372	940
Ending balance	289,943	255,760	281,608
Commercial
Allowance for loan losses
Beginning balance	16,732	13,339	13,339
Charge-offs	(3,360)	(1,255)
Recoveries	80	84
Provision (credit)	2,650	1,949
Provision for acquired loans	0	88
Ending balance	16,102	14,205	16,732
Loans individually evaluated for impairment	2,632	1,860
Loans collectively evaluated for impairment	13,296	12,165
Acquired loans evaluated for impairment	174	180
Ending balance	16,102	14,205
Loans individually evaluated for impairment	16,993	16,767
Loans collectively evaluated for impairment	1,361,517	1,175,911
Acquired nonimpaired loans	107,183	141,933
Acquired impaired loans	3,870	5,256	4,156
Ending balance	1,489,563	1,339,867	1,464,554
Residential
Allowance for loan losses
Beginning balance	1,798	2,059	2,059
Charge-offs	0	(11)
Recoveries	14	120
Provision (credit)	(129)	(114)
Provision for acquired loans	(3)	0
Ending balance	1,680	2,054	1,798
Loans individually evaluated for impairment	643	887
Loans collectively evaluated for impairment	1,001	1,144
Acquired loans evaluated for impairment	35	23
Ending balance	1,679	2,054
Loans individually evaluated for impairment	12,861	14,280
Loans collectively evaluated for impairment	145,753	158,430
Acquired nonimpaired loans	67,722	85,679
Acquired impaired loans	777	866	784
Ending balance	227,113	259,255	233,487
Consumer
Allowance for loan losses
Beginning balance	7,895	6,012	6,012
Charge-offs	(462)	(1,143)
Recoveries	207	305
Provision (credit)	548	1,080
Provision for acquired loans	0	(13)
Ending balance	8,188	6,241	7,895
Loans individually evaluated for impairment	186	194
Loans collectively evaluated for impairment	7,994	6,042
Acquired loans evaluated for impairment	9	5
Ending balance	8,189	6,241
Loans individually evaluated for impairment	7,677	9,029
Loans collectively evaluated for impairment	541,644	405,285
Acquired nonimpaired loans	33,152	50,321
Acquired impaired loans	249	263	247
Ending balance	$ 582,722	$ 464,898	$ 558,493