ALLOWANCE FOR LOAN LOSSES AND CREDIT QUALITY INFORMATION - Schedule of Allowance for Loan Losses and Loan Balances (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Allowance for loan losses
Beginning balance	$ 41,037	$ 40,005	$ 40,599	$ 39,751	$ 39,751
Charge-offs	(3,803)	(3,752)	(10,646)	(8,849)
Recoveries	862	1,052	1,995	2,398
Provision (credit)	3,834	2,823	9,940	6,280
Provision (credit) for acquired loans	(118)	73	(76)	621
Ending balance	41,812	40,201	41,812	40,201	40,599
Loans individually evaluated for impairment	5,164	2,407	5,164	2,407
Loans collectively evaluated for impairment	36,419	37,252	36,419	37,252
Acquired loans evaluated for impairment			229	542
Ending balance	41,812	40,201	41,812	40,201
Loans individually evaluated for impairment	51,879	48,511	51,879	48,511
Loans collectively evaluated for impairment	4,384,467	4,026,672	4,384,467	4,026,672
Acquired nonimpaired loans	465,438	596,466	465,438	596,466	565,500
Acquired impaired loans			17,493	24,582	21,654
Ending balance	4,919,277	4,696,231	4,919,277	4,696,231	4,805,094
Reverse mortgage, fair value	16,600	21,400	16,600	21,400	19,800
Performing TDRs			15,192	14,900	20,061
Owner-occupied commercial
Allowance for loan losses
Beginning balance	5,284	5,816	5,422	6,588	6,588
Charge-offs	0	(104)	(351)	(296)
Recoveries	16	12	28	120
Provision (credit)	273	(96)	419	(802)
Provision (credit) for acquired loans	0	104	55	122
Ending balance	5,573	5,732	5,573	5,732	5,422
Loans individually evaluated for impairment	9	0	9	0
Loans collectively evaluated for impairment	5,546	5,699	5,546	5,699
Acquired loans evaluated for impairment			18	33
Ending balance	5,573	5,732	5,573	5,732
Loans individually evaluated for impairment	2,829	3,346	2,829	3,346
Loans collectively evaluated for impairment	958,356	941,296	958,356	941,296
Acquired nonimpaired loans	119,403	144,710	119,403	144,710
Acquired impaired loans			4,816	7,401	5,803
Ending balance	1,085,404	1,096,753	1,085,404	1,096,753	1,079,247
Commercial mortgages
Allowance for loan losses
Beginning balance	6,951	7,335	5,891	8,915	8,915
Charge-offs	0	(1,196)	(48)	(1,702)
Recoveries	52	16	189	69
Provision (credit)	(598)	(231)	356	(1,602)
Provision (credit) for acquired loans	(21)	(5)	(4)	239
Ending balance	6,384	5,919	6,384	5,919	5,891
Loans individually evaluated for impairment	0	131	0	131
Loans collectively evaluated for impairment	6,300	5,638	6,300	5,638
Acquired loans evaluated for impairment			84	150
Ending balance	6,384	5,919	6,384	5,919
Loans individually evaluated for impairment	6,502	9,012	6,502	9,012
Loans collectively evaluated for impairment	961,345	943,699	961,345	943,699
Acquired nonimpaired loans	156,483	194,394	156,483	194,394
Acquired impaired loans			8,951	9,969	9,724
Ending balance	1,133,281	1,157,074	1,133,281	1,157,074	1,187,705
Construction
Allowance for loan losses
Beginning balance	3,289	3,432	2,861	2,838	2,838
Charge-offs	(1,475)	(215)	(1,475)	(346)
Recoveries	1	301	3	305
Provision (credit)	1,657	427	2,106	1,056
Provision (credit) for acquired loans	0	(28)	(23)	64
Ending balance	3,472	3,917	3,472	3,917	2,861
Loans individually evaluated for impairment	444	0	444	0
Loans collectively evaluated for impairment	3,019	3,881	3,019	3,881
Acquired loans evaluated for impairment			9	36
Ending balance	3,472	3,917	3,472	3,917
Loans individually evaluated for impairment	2,903	1,839	2,903	1,839
Loans collectively evaluated for impairment	322,822	271,447	322,822	271,447
Acquired nonimpaired loans	7,025	19,085	7,025	19,085
Acquired impaired loans			737	946	940
Ending balance	333,487	293,317	333,487	293,317	281,608
Commercial
Allowance for loan losses
Beginning balance	15,842	14,224	16,732	13,339	13,339
Charge-offs	(1,761)	(1,603)	(6,861)	(3,787)
Recoveries	621	417	1,060	820
Provision (credit)	1,947	2,128	5,730	4,597
Provision (credit) for acquired loans	(82)	(7)	(94)	190
Ending balance	16,567	15,159	16,567	15,159	16,732
Loans individually evaluated for impairment	3,970	1,220	3,970	1,220
Loans collectively evaluated for impairment	12,517	13,646	12,517	13,646
Acquired loans evaluated for impairment			80	293
Ending balance	16,567	15,159	16,567	15,159
Loans individually evaluated for impairment	19,910	12,845	19,910	12,845
Loans collectively evaluated for impairment	1,387,143	1,249,027	1,387,143	1,249,027
Acquired nonimpaired loans	97,552	120,987	97,552	120,987
Acquired impaired loans			2,070	5,235	4,156
Ending balance	1,506,675	1,388,094	1,506,675	1,388,094	1,464,554
Residential
Allowance for loan losses
Beginning balance	1,519	2,050	1,798	2,059	2,059
Charge-offs	0	(59)	(54)	(112)
Recoveries	28	11	117	141
Provision (credit)	(71)	(49)	(382)	(146)
Provision (credit) for acquired loans	(1)	9	(4)	20
Ending balance	1,475	1,962	1,475	1,962	1,798
Loans individually evaluated for impairment	570	858	570	858
Loans collectively evaluated for impairment	870	1,078	870	1,078
Acquired loans evaluated for impairment			35	26
Ending balance	1,475	1,962	1,475	1,962
Loans individually evaluated for impairment	11,479	14,060	11,479	14,060
Loans collectively evaluated for impairment	134,074	148,715	134,074	148,715
Acquired nonimpaired loans	60,407	77,154	60,407	77,154
Acquired impaired loans			766	788	784
Ending balance	206,726	240,717	206,726	240,717	233,487
Consumer
Allowance for loan losses
Beginning balance	8,152	7,148	7,895	6,012	6,012
Charge-offs	(567)	(575)	(1,857)	(2,606)
Recoveries	144	295	598	943
Provision (credit)	626	644	1,711	3,177
Provision (credit) for acquired loans	(14)	0	(6)	(14)
Ending balance	8,341	7,512	8,341	7,512	7,895
Loans individually evaluated for impairment	171	198	171	198
Loans collectively evaluated for impairment	8,167	7,310	8,167	7,310
Acquired loans evaluated for impairment			3	4
Ending balance	8,341	7,512	8,341	7,512
Loans individually evaluated for impairment	8,256	7,409	8,256	7,409
Loans collectively evaluated for impairment	620,727	472,488	620,727	472,488
Acquired nonimpaired loans	24,568	40,136	24,568	40,136
Acquired impaired loans			153	243	247
Ending balance	$ 653,704	$ 520,276	$ 653,704	$ 520,276	$ 558,493