LOAN PORTFOLIO AND CREDIT QUALITY - Loans by Credit Quality Indicator (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment [Line Items]
2020	$ 1,446,964
2019	1,171,069
2018	787,841
2017	784,080
2016	889,143
Prior	1,460,828
Revolving	515,053
Revolving converted to term	49,331
Total loans	7,104,309	$ 6,976,704
Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
2020	97,772
2019	83,726
2018	73,111
2017	14,052
2016	18,785
Prior	46,404
Revolving	204,858
Revolving converted to term	19,635
Total loans	558,343	694,034
Paycheck Protection Program
Financing Receivable, Recorded Investment [Line Items]
2020	312,356
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving	0
Revolving converted to term	0
Total loans	312,356	0
Commercial tax-exempt
Financing Receivable, Recorded Investment [Line Items]
2020	53,225
2019	20,586
2018	40,451
2017	24,624
2016	102,133
Prior	198,107
Revolving	0
Revolving converted to term	3,033
Total loans	442,159	447,927
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
2020	336,427
2019	530,844
2018	273,610
2017	337,898
2016	412,836
Prior	722,977
Revolving	126,593
Revolving converted to term	16,190
Total loans	2,757,375	2,551,274
Construction and land
Financing Receivable, Recorded Investment [Line Items]
2020	43,042
2019	63,914
2018	33,730
2017	16,288
2016	2,230
Prior	0
Revolving	0
Revolving converted to term	0
Total loans	159,204	225,983
Residential
Financing Receivable, Recorded Investment [Line Items]
2020	603,414
2019	471,841
2018	366,662
2017	391,218
2016	352,392
Prior	491,937
Revolving	0
Revolving converted to term	0
Total loans	2,677,464	2,839,155
Home equity
Financing Receivable, Recorded Investment [Line Items]
2020	0
2019	0
2018	252
2017	0
2016	686
Prior	829
Revolving	65,124
Revolving converted to term	10,473
Total loans	77,364	83,657
Consumer and other
Financing Receivable, Recorded Investment [Line Items]
2020	728
2019	158
2018	25
2017	0
2016	81
Prior	574
Revolving	118,478
Revolving converted to term	0
Total loans	120,044	134,674
Pass
Financing Receivable, Recorded Investment [Line Items]
2020	1,420,600
2019	1,098,988
2018	751,286
2017	751,572
2016	851,145
Prior	1,371,548
Revolving	500,958
Revolving converted to term	40,241
Total loans	6,786,338
Pass | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
2020	96,230
2019	80,949
2018	65,506
2017	13,378
2016	17,972
Prior	43,592
Revolving	191,252
Revolving converted to term	10,801
Total loans	519,680
Pass | Paycheck Protection Program
Financing Receivable, Recorded Investment [Line Items]
2020	312,356
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving	0
Revolving converted to term	0
Total loans	312,356
Pass | Commercial tax-exempt
Financing Receivable, Recorded Investment [Line Items]
2020	53,225
2019	20,586
2018	40,451
2017	24,624
2016	102,133
Prior	190,798
Revolving	0
Revolving converted to term	3,033
Total loans	434,850
Pass | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
2020	311,605
2019	462,144
2018	247,228
2017	308,437
2016	375,713
Prior	657,563
Revolving	126,544
Revolving converted to term	16,190
Total loans	2,505,424
Pass | Construction and land
Financing Receivable, Recorded Investment [Line Items]
2020	43,042
2019	63,914
2018	31,434
2017	16,288
2016	2,230
Prior	0
Revolving	0
Revolving converted to term	0
Total loans	156,908
Pass | Residential
Financing Receivable, Recorded Investment [Line Items]
2020	603,414
2019	471,237
2018	366,390
2017	388,845
2016	352,330
Prior	478,468
Revolving	0
Revolving converted to term	0
Total loans	2,660,684
Pass | Home equity
Financing Receivable, Recorded Investment [Line Items]
2020	0
2019	0
2018	252
2017	0
2016	686
Prior	553
Revolving	64,985
Revolving converted to term	10,217
Total loans	76,693
Pass | Consumer and other
Financing Receivable, Recorded Investment [Line Items]
2020	728
2019	158
2018	25
2017	0
2016	81
Prior	574
Revolving	118,177
Revolving converted to term	0
Total loans	119,743
Special Mention
Financing Receivable, Recorded Investment [Line Items]
2020	22,019
2019	16,264
2018	15,686
2017	30,135
2016	37,123
Prior	61,537
Revolving	4,211
Revolving converted to term	262
Total loans	187,237
Special Mention | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
2020	358
2019	2,413
2018	1,008
2017	674
2016	0
Prior	2,688
Revolving	3,911
Revolving converted to term	262
Total loans	11,314
Special Mention | Commercial tax-exempt
Financing Receivable, Recorded Investment [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	2,806
Revolving	0
Revolving converted to term	0
Total loans	2,806
Special Mention | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
2020	21,661
2019	13,851
2018	12,382
2017	29,461
2016	37,123
Prior	56,043
Revolving	0
Revolving converted to term	0
Total loans	170,521
Special Mention | Construction and land
Financing Receivable, Recorded Investment [Line Items]
2020	0
2019	0
2018	2,296
2017	0
2016	0
Prior	0
Revolving	0
Revolving converted to term	0
Total loans	2,296
Special Mention | Consumer and other
Financing Receivable, Recorded Investment [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving	300
Revolving converted to term	0
Total loans	300
Substandard
Financing Receivable, Recorded Investment [Line Items]
2020	4,345
2019	49,860
2018	20,247
2017	0
2016	0
Prior	16,984
Revolving	8,683
Revolving converted to term	6,764
Total loans	106,883
Substandard | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
2020	1,184
2019	223
2018	6,247
2017	0
2016	0
Prior	110
Revolving	8,683
Revolving converted to term	6,508
Total loans	22,955
Substandard | Commercial tax-exempt
Financing Receivable, Recorded Investment [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	4,503
Revolving	0
Revolving converted to term	0
Total loans	4,503
Substandard | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
2020	3,161
2019	49,637
2018	14,000
2017	0
2016	0
Prior	9,371
Revolving	0
Revolving converted to term	0
Total loans	76,169
Substandard | Residential
Financing Receivable, Recorded Investment [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	3,000
Revolving	0
Revolving converted to term	0
Total loans	3,000
Substandard | Home equity
Financing Receivable, Recorded Investment [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving	0
Revolving converted to term	256
Total loans	256
Performing Financial Instruments | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,786,338	6,850,653
Performing Financial Instruments | Pass | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total loans	519,680	656,364
Performing Financial Instruments | Pass | Paycheck Protection Program
Financing Receivable, Recorded Investment [Line Items]
Total loans	312,356
Performing Financial Instruments | Pass | Commercial tax-exempt
Financing Receivable, Recorded Investment [Line Items]
Total loans	434,850	436,721
Performing Financial Instruments | Pass | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,505,424	2,495,702
Performing Financial Instruments | Pass | Construction and land
Financing Receivable, Recorded Investment [Line Items]
Total loans	156,908	225,526
Performing Financial Instruments | Pass | Residential
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,660,684	2,820,909
Performing Financial Instruments | Pass | Home equity
Financing Receivable, Recorded Investment [Line Items]
Total loans	76,693	81,060
Performing Financial Instruments | Pass | Consumer and other
Financing Receivable, Recorded Investment [Line Items]
Total loans	119,743	134,371
Performing Financial Instruments | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	187,237	52,026
Performing Financial Instruments | Special Mention | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total loans	11,314	12,101
Performing Financial Instruments | Special Mention | Paycheck Protection Program
Financing Receivable, Recorded Investment [Line Items]
Total loans	0
Performing Financial Instruments | Special Mention | Commercial tax-exempt
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,806	7,154
Performing Financial Instruments | Special Mention | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	170,521	32,014
Performing Financial Instruments | Special Mention | Construction and land
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,296	457
Performing Financial Instruments | Special Mention | Residential
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Performing Financial Instruments | Special Mention | Home equity
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Performing Financial Instruments | Special Mention | Consumer and other
Financing Receivable, Recorded Investment [Line Items]
Total loans	300	300
Performing Financial Instruments | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	106,883	57,922
Performing Financial Instruments | Substandard | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total loans	22,955	24,987
Performing Financial Instruments | Substandard | Paycheck Protection Program
Financing Receivable, Recorded Investment [Line Items]
Total loans	0
Performing Financial Instruments | Substandard | Commercial tax-exempt
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,503	4,052
Performing Financial Instruments | Substandard | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	76,169	23,558
Performing Financial Instruments | Substandard | Construction and land
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Performing Financial Instruments | Substandard | Residential
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,000	4,253
Performing Financial Instruments | Substandard | Home equity
Financing Receivable, Recorded Investment [Line Items]
Total loans	256	1,072
Performing Financial Instruments | Substandard | Consumer and other
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Nonperforming Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
2020	0
2019	5,957
2018	622
2017	2,373
2016	875
Prior	10,759
Revolving	1,201
Revolving converted to term	2,064
Total loans	23,851
Nonperforming Financial Instruments | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
2020	0
2019	141
2018	350
2017	0
2016	813
Prior	14
Revolving	1,012
Revolving converted to term	2,064
Total loans	4,394
Nonperforming Financial Instruments | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
2020	0
2019	5,212
2018	0
2017	0
2016	0
Prior	0
Revolving	49
Revolving converted to term	0
Total loans	5,261
Nonperforming Financial Instruments | Residential
Financing Receivable, Recorded Investment [Line Items]
2020	0
2019	604
2018	272
2017	2,373
2016	62
Prior	10,469
Revolving	0
Revolving converted to term	0
Total loans	13,780
Nonperforming Financial Instruments | Home equity
Financing Receivable, Recorded Investment [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	276
Revolving	139
Revolving converted to term	0
Total loans	415
Nonperforming Financial Instruments | Consumer and other
Financing Receivable, Recorded Investment [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving	1
Revolving converted to term	0
Total loans	1
Nonperforming Financial Instruments | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	23,851	16,103
Nonperforming Financial Instruments | Substandard | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,394	582
Nonperforming Financial Instruments | Substandard | Paycheck Protection Program
Financing Receivable, Recorded Investment [Line Items]
Total loans	0
Nonperforming Financial Instruments | Substandard | Commercial tax-exempt
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Nonperforming Financial Instruments | Substandard | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,261	0
Nonperforming Financial Instruments | Substandard | Construction and land
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Nonperforming Financial Instruments | Substandard | Residential
Financing Receivable, Recorded Investment [Line Items]
Total loans	13,780	13,993
Nonperforming Financial Instruments | Substandard | Home equity
Financing Receivable, Recorded Investment [Line Items]
Total loans	415	1,525
Nonperforming Financial Instruments | Substandard | Consumer and other
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 1	$ 3