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ACQUISITIONS (Details) shares in Thousands, $ in Thousands													3 Months Ended				12 Months Ended
ACQUISITIONS (Details) shares in Thousands, $ in Thousands	Oct. 21, 2019 USD ($) Branch County	Sep. 18, 2019 USD ($)	Jul. 12, 2019 USD ($) County Branch	Jan. 02, 2019 USD ($)	Apr. 02, 2018 USD ($)	Jan. 02, 2018 USD ($)	Dec. 04, 2017 USD ($) shares	Nov. 17, 2017 USD ($)	May 12, 2017 USD ($) Branch shares	Mar. 01, 2017 USD ($)	Feb. 03, 2017 USD ($)	Jan. 01, 2017 USD ($)	Dec. 31, 2019 USD ($) County	Dec. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) County	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 02, 2017 USD ($)
Acquisitions [Abstract]
Cash		$ 500	$ 95,034														$ 4,653	$ 1,737	$ 107,414
Purchase price of acquisition			95,034
Other liabilities			3,259
Goodwill			40,307										$ 773,810	$ 733,503			773,810	733,503	734,430
Investment securities			39,770
Increase in deposits																	$ 104,435	(122,049)	$ (79,940)
Vermont [Member]
Acquisitions [Abstract]
Number of counties where the bank has facilities \| County													12				12
Massachusetts [Member]
Acquisitions [Abstract]
Number of counties where the bank has facilities \| County													1				1
New York [Member]
Acquisitions [Abstract]
Number of counties where the bank has facilities \| County													40				40
Customer Lists [Member]
Acquisitions [Abstract]
Intangibles		$ 500
Other Intangibles [Member]
Acquisitions [Abstract]
Intangibles			1,650
Steuben Trust Corporation [Member]
Acquisitions [Abstract]
Cash	$ 104,400
Steuben Trust Corporation [Member] \| Western New York State [Member]
Acquisitions [Abstract]
Number of branch locations \| Branch	14
Number of new counties where the bank has extended footprints \| Branch	2
Number of counties where the bank has facilities \| County	4
Kinderhook Bank Corp [Member]
Acquisitions [Abstract]
Cash			93,384
Purchase price of acquisition			$ 93,384
Number of branch locations \| Branch			11
Other liabilities			$ 3,259
Goodwill			40,307
Investment securities			$ 39,770
Increase (decrease) in other liabilities as a result of reclassification and adjustment													$ 40
Increase (decrease) in goodwill as a result of fair value adjustment													200
Increase in deposits													80
Decrease in loans													(50)
Decrease in other assets													(40)
Increased in accrued interest and fees receivable													$ 10
Kinderhook Bank Corp [Member] \| Upstate New York [Member]
Acquisitions [Abstract]
Number of counties where the bank has facilities \| County			5
Assets acquired			$ 642,800
Revenue earned			10,600
Direct expenses			4,700
Kinderhook Bank Corp [Member] \| Other Intangibles [Member]
Acquisitions [Abstract]
Intangibles			$ 0
Wealth Resources Network, Inc [Member]
Acquisitions [Abstract]
Cash				$ 1,200
Wealth Resources Network, Inc [Member] \| Customer Lists [Member]
Acquisitions [Abstract]
Intangibles				$ 1,200
HR Consultants (SA), LLC [Member]
Acquisitions [Abstract]
Cash					$ 300
HR Consultants (SA), LLC [Member] \| Customer Lists [Member]
Acquisitions [Abstract]
Intangibles					$ 300
Penna & Associates Agency, Inc. [Member]
Acquisitions [Abstract]
Cash						$ 800
Goodwill						300
Penna & Associates Agency, Inc. [Member] \| Customer Lists [Member]
Acquisitions [Abstract]
Intangibles						300
Styles Bridges Associates [Member]
Acquisitions [Abstract]
Cash						700
Styles Bridges Associates [Member] \| Customer Lists [Member]
Acquisitions [Abstract]
Intangibles						$ 700
Gordon B. Roberts Agency, Inc [Member]
Acquisitions [Abstract]
Cash							$ 1,350
Purchase price of acquisition							$ 3,700
Issuance of common shares (in shares) \| shares							40
Assets acquired							$ 600
Other liabilities							600
Goodwill							2,100
Increase (decrease) in other liabilities as a result of reclassification and adjustment														20	$ (90)
Increase (decrease) in goodwill as a result of fair value adjustment														$ 20	$ (90)
Gordon B. Roberts Agency, Inc [Member] \| Other Intangibles [Member]
Acquisitions [Abstract]
Intangibles							$ 1,600
Northeast Capital Management, Inc [Member]
Acquisitions [Abstract]
Cash								$ 1,200
Northeast Capital Management, Inc [Member] \| Customer Lists [Member]
Acquisitions [Abstract]
Intangibles								$ 1,200
Dryfoos [Member]
Acquisitions [Abstract]
Cash										$ 3,000
Goodwill										1,700
Dryfoos [Member] \| Other Intangibles [Member]
Acquisitions [Abstract]
Intangibles										$ 1,700
BAS [Member]
Acquisitions [Abstract]
Cash												$ 1,200
BAS [Member] \| Other Intangibles [Member]
Acquisitions [Abstract]
Intangibles																				$ 1,200
Merchants Bancshares, Inc. [Member]
Acquisitions [Abstract]
Cash									$ 82,898
Purchase price of acquisition									$ 345,152
Issuance of common shares (in shares) \| shares									4,680
Assets acquired									$ 2,000,000
Other liabilities									11,750
Goodwill									189,046
Investment securities									$ 370,648
Revenue earned																	$ 55,800	61,200
Direct expenses																	$ 29,400	30,800
Merchants Bancshares, Inc. [Member] \| Vermont [Member]
Acquisitions [Abstract]
Number of branch locations \| Branch									31
Merchants Bancshares, Inc. [Member] \| Massachusetts [Member]
Acquisitions [Abstract]
Number of branch locations \| Branch									1
Merchants Bancshares, Inc. [Member] \| Other Intangibles [Member]
Acquisitions [Abstract]
Intangibles									$ 2,857
NRS [Member]
Acquisitions [Abstract]
Cash											$ 70,073
Purchase price of acquisition											148,556
Tangible assets, net											$ 36,100
Factor of straight-line rate of depreciation used in declining balance depreciation method											150.00%
Period of amortization																	10 years
Deferred tax liability											$ 23,000
Other liabilities											26,828
Goodwill											75,256
Investment securities											20,294
Revenue earned																	$ 44,000	40,600
Direct expenses																	$ 23,900	$ 24,600
Increase (decrease) in other liabilities as a result of reclassification and adjustment																$ (1,200)
Increase (decrease) in goodwill as a result of fair value adjustment																$ (1,200)
NRS [Member] \| Other Intangibles [Member]
Acquisitions [Abstract]
Intangibles											$ 60,200