ALLOWANCE FOR LOAN LOSSES AND CREDIT QUALITY INFORMATION - Schedule of Allowance for Loan Losses and Loan Balances (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Allowance for loan losses
Beginning balance	$ 40,810	$ 39,826	$ 40,599	$ 39,751	$ 39,751
Charge-offs	(2,963)	(2,378)	(6,844)	(5,097)
Recoveries	692	714	1,134	1,346
Provision (credit)	2,451	1,343	6,106	3,457
Provision for acquired loans	47	500	42	548
Ending balance	41,037	40,005	41,037	40,005	40,599
Loans individually evaluated for impairment	3,631	4,642	3,631	4,642
Loans collectively evaluated for impairment	37,080	34,775	37,080	34,775
Acquired loans evaluated for impairment			326	588
Ending balance	41,037	40,005	41,037	40,005
Loans individually evaluated for impairment	52,529	56,445	52,529	56,445
Loans collectively evaluated for impairment	4,329,312	3,888,382	4,329,312	3,888,382
Acquired nonimpaired loans	501,057	629,797	501,057	629,797	565,500
Acquired impaired loans			18,723	31,147	21,654
Ending balance	4,901,621	4,605,771	4,901,621	4,605,771	4,805,094
Owner-occupied commercial
Allowance for loan losses
Beginning balance	5,359	6,030	5,422	6,588	6,588
Charge-offs	(341)	0	(351)	(192)
Recoveries	7	33	12	108
Provision (credit)	204	(265)	146	(706)
Provision for acquired loans	55	18	55	18
Ending balance	5,284	5,816	5,284	5,816	5,422
Loans individually evaluated for impairment	0	0	0	0
Loans collectively evaluated for impairment	5,266	5,783	5,266	5,783
Acquired loans evaluated for impairment			18	33
Ending balance	5,284	5,816	5,284	5,816
Loans individually evaluated for impairment	3,224	3,432	3,224	3,432
Loans collectively evaluated for impairment	952,627	945,919	952,627	945,919
Acquired nonimpaired loans	125,129	148,686	125,129	148,686
Acquired impaired loans			4,915	11,839	5,803
Ending balance	1,085,895	1,109,876	1,085,895	1,109,876	1,079,247
Commercial mortgages
Allowance for loan losses
Beginning balance	6,617	8,335	5,891	8,915	8,915
Charge-offs	0	(402)	(48)	(506)
Recoveries	3	7	137	53
Provision (credit)	337	(853)	954	(1,371)
Provision for acquired loans	(6)	248	17	244
Ending balance	6,951	7,335	6,951	7,335	5,891
Loans individually evaluated for impairment	63	1,464	63	1,464
Loans collectively evaluated for impairment	6,804	5,714	6,804	5,714
Acquired loans evaluated for impairment			84	157
Ending balance	6,951	7,335	6,951	7,335
Loans individually evaluated for impairment	6,737	10,941	6,737	10,941
Loans collectively evaluated for impairment	972,684	921,069	972,684	921,069
Acquired nonimpaired loans	172,082	204,809	172,082	204,809
Acquired impaired loans			9,151	10,187	9,724
Ending balance	1,160,654	1,147,006	1,160,654	1,147,006	1,187,705
Construction
Allowance for loan losses
Beginning balance	2,864	2,961	2,861	2,838	2,838
Charge-offs	0	(117)	0	(131)
Recoveries	1	2	1	4
Provision (credit)	422	471	450	629
Provision for acquired loans	2	115	(23)	92
Ending balance	3,289	3,432	3,289	3,432	2,861
Loans individually evaluated for impairment	593	772	593	772
Loans collectively evaluated for impairment	2,687	2,595	2,687	2,595
Acquired loans evaluated for impairment			9	65
Ending balance	3,289	3,432	3,289	3,432
Loans individually evaluated for impairment	5,557	4,808	5,557	4,808
Loans collectively evaluated for impairment	283,480	252,637	283,480	252,637
Acquired nonimpaired loans	7,352	20,340	7,352	20,340
Acquired impaired loans			731	2,021	940
Ending balance	297,120	279,806	297,120	279,806	281,608
Commercial
Allowance for loan losses
Beginning balance	16,102	14,205	16,732	13,339	13,339
Charge-offs	(1,740)	(929)	(5,100)	(2,184)
Recoveries	359	319	439	403
Provision (credit)	1,133	520	3,783	2,469
Provision for acquired loans	(12)	109	(12)	197
Ending balance	15,842	14,224	15,842	14,224	16,732
Loans individually evaluated for impairment	2,208	1,342	2,208	1,342
Loans collectively evaluated for impairment	13,472	12,587	13,472	12,587
Acquired loans evaluated for impairment			162	295
Ending balance	15,842	14,224	15,842	14,224
Loans individually evaluated for impairment	17,015	14,845	17,015	14,845
Loans collectively evaluated for impairment	1,404,662	1,171,831	1,404,662	1,171,831
Acquired nonimpaired loans	101,532	130,986	101,532	130,986
Acquired impaired loans			2,904	6,002	4,156
Ending balance	1,526,113	1,323,664	1,526,113	1,323,664	1,464,554
Residential
Allowance for loan losses
Beginning balance	1,680	2,054	1,798	2,059	2,059
Charge-offs	(54)	(42)	(54)	(53)
Recoveries	75	10	91	130
Provision (credit)	(182)	17	(313)	(97)
Provision for acquired loans	0	11	(3)	11
Ending balance	1,519	2,050	1,519	2,050	1,798
Loans individually evaluated for impairment	592	869	592	869
Loans collectively evaluated for impairment	891	1,147	891	1,147
Acquired loans evaluated for impairment			36	34
Ending balance	1,519	2,050	1,519	2,050
Loans individually evaluated for impairment	12,282	14,023	12,282	14,023
Loans collectively evaluated for impairment	134,323	157,653	134,323	157,653
Acquired nonimpaired loans	65,723	80,074	65,723	80,074
Acquired impaired loans			771	852	784
Ending balance	213,099	252,602	213,099	252,602	233,487
Consumer
Allowance for loan losses
Beginning balance	8,188	6,241	7,895	6,012	6,012
Charge-offs	(828)	(888)	(1,291)	(2,031)
Recoveries	247	343	454	648
Provision (credit)	537	1,453	1,086	2,533
Provision for acquired loans	8	(1)	8	(14)
Ending balance	8,152	7,148	8,152	7,148	7,895
Loans individually evaluated for impairment	175	195	175	195
Loans collectively evaluated for impairment	7,960	6,949	7,960	6,949
Acquired loans evaluated for impairment			17	4
Ending balance	8,152	7,148	8,152	7,148
Loans individually evaluated for impairment	7,714	8,396	7,714	8,396
Loans collectively evaluated for impairment	581,536	439,273	581,536	439,273
Acquired nonimpaired loans	29,239	44,902	29,239	44,902
Acquired impaired loans			251	246	247
Ending balance	$ 618,740	$ 492,817	$ 618,740	$ 492,817	$ 558,493