ALLOWANCE FOR CREDIT LOSSES AND CREDIT QUALITY - Allowance for loan losses by portfolio segment (Details) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020		Jun. 30, 2021		Jun. 30, 2020
Post adoption of ASC 326
Beginning balance		$ 71,844			$ 61,408
Provision for credit losses	[1]	(1,700)			(223)
Loans charged-off		(267)			(503)
Recoveries		1,836			1,981
Net loans charged-off		1,569			1,478
Ending balance	[2]	71,713			71,713
Prior to adoption of ASC 326
Beginning balance			$ 53,609		61,408	[2]	$ 50,652
Provision for credit losses			5,500	[1]			12,100	[3]
Loans charged-off			(411)				(4,260)
Recoveries			321				527
Net loans charged-off			(90)				(3,733)
Ending balance			59,019				59,019
Impact of ASC 326 Adoption | ASU 2016-13
Post adoption of ASC 326
Beginning balance					9,050
Commercial and industrial loans
Prior to adoption of ASC 326
Beginning balance			24,739		28,333		25,789
Provision for credit losses			2,062				4,690
Loans charged-off			(296)				(4,031)
Recoveries			239				296
Net loans charged-off			(57)				(3,735)
Ending balance			26,744				26,744
Commercial and industrial loans | Commercial Portfolio Segment
Post adoption of ASC 326
Beginning balance		32,052			28,333
Provision for credit losses		(187)			(727)
Loans charged-off		(162)			(249)
Recoveries		1,427			1,461
Net loans charged-off		1,265			1,212
Ending balance		33,130			33,130
Commercial and industrial loans | Impact of ASC 326 Adoption | ASU 2016-13 | Commercial Portfolio Segment
Post adoption of ASC 326
Beginning balance					4,312
Commercial real estate and multi-family residential loans
Prior to adoption of ASC 326
Beginning balance			18,658		22,907		15,796
Provision for credit losses			2,387				5,137
Loans charged-off			0				0
Recoveries			18				130
Net loans charged-off			18				130
Ending balance			21,063				21,063
Commercial real estate and multi-family residential loans | Commercial Portfolio Segment
Post adoption of ASC 326
Beginning balance		29,445			22,907
Provision for credit losses		(1,160)			1,125
Loans charged-off		0			(71)
Recoveries		6			14
Net loans charged-off		6			(57)
Ending balance		28,291			28,291
Commercial real estate and multi-family residential loans | Impact of ASC 326 Adoption | ASU 2016-13 | Commercial Portfolio Segment
Post adoption of ASC 326
Beginning balance					4,316
Agri-business and agricultural loans
Prior to adoption of ASC 326
Beginning balance			3,704		3,043		3,869
Provision for credit losses			(299)				(466)
Loans charged-off			0				0
Recoveries			3				5
Net loans charged-off			3				5
Ending balance			3,408				3,408
Agri-business and agricultural loans | Commercial Portfolio Segment
Post adoption of ASC 326
Beginning balance		3,901			3,043
Provision for credit losses		(291)			(493)
Loans charged-off		0			0
Recoveries		320			320
Net loans charged-off		320			320
Ending balance		3,930			3,930
Agri-business and agricultural loans | Impact of ASC 326 Adoption | ASU 2016-13 | Commercial Portfolio Segment
Post adoption of ASC 326
Beginning balance					1,060
Other Commercial
Prior to adoption of ASC 326
Beginning balance			521		416		447
Provision for credit losses			21				95
Loans charged-off			0				0
Recoveries			0				0
Net loans charged-off			0				0
Ending balance			542				542
Other Commercial | Commercial Portfolio Segment
Post adoption of ASC 326
Beginning balance		1,172			416
Provision for credit losses		126			(59)
Loans charged-off		0			0
Recoveries		0			0
Net loans charged-off		0			0
Ending balance		1,298			1,298
Other Commercial | Impact of ASC 326 Adoption | ASU 2016-13 | Commercial Portfolio Segment
Post adoption of ASC 326
Beginning balance					941
Consumer 1-4 family loans
Prior to adoption of ASC 326
Beginning balance			2,475		2,619		2,086
Provision for credit losses			953				1,348
Loans charged-off			0				(13)
Recoveries			6				13
Net loans charged-off			6				0
Ending balance			3,434				3,434
Consumer 1-4 family loans | Consumer Portfolio Segment
Post adoption of ASC 326
Beginning balance		3,384			2,619
Provision for credit losses		(221)			(454)
Loans charged-off		(32)			(38)
Recoveries		34			85
Net loans charged-off		2			47
Ending balance		3,165			3,165
Consumer 1-4 family loans | Impact of ASC 326 Adoption | ASU 2016-13 | Consumer Portfolio Segment
Post adoption of ASC 326
Beginning balance					953
Other Consumer
Prior to adoption of ASC 326
Beginning balance			447		951		345
Provision for credit losses			387				562
Loans charged-off			(115)				(216)
Recoveries			55				83
Net loans charged-off			(60)				(133)
Ending balance			774				774
Other Consumer | Consumer Portfolio Segment
Post adoption of ASC 326
Beginning balance		1,293			951
Provision for credit losses		124			137
Loans charged-off		(73)			(145)
Recoveries		49			101
Net loans charged-off		(24)			(44)
Ending balance		1,393			1,393
Other Consumer | Impact of ASC 326 Adoption | ASU 2016-13 | Consumer Portfolio Segment
Post adoption of ASC 326
Beginning balance					349
Unallocated
Post adoption of ASC 326
Beginning balance		597			3,139
Provision for credit losses		(91)			248
Loans charged-off		0			0
Recoveries		0			0
Net loans charged-off		0			0
Ending balance		$ 506			506
Prior to adoption of ASC 326
Beginning balance			3,065		3,139		2,320
Provision for credit losses			(11)				734
Loans charged-off			0				0
Recoveries			0				0
Net loans charged-off			0				0
Ending balance			$ 3,054				$ 3,054
Unallocated | Impact of ASC 326 Adoption | ASU 2016-13
Post adoption of ASC 326
Beginning balance					$ (2,881)

[1]  Beginning January 1, 2021 calculation is based on the current expected credit loss methodology. Prior to January 1, 2021 calculation was based on the incurred loss methodology.

[2] Beginning January 1, 2021 calculation is based on the current expected credit loss methodology. Prior to January 1, 2021 calculation was based on the incurred loss methodology.

[3] Beginning January 1, 2021 calculation is based on the current expected credit loss methodology. Prior to January 1, 2021 calculation was based on the incurred loss methodology.