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ACQUISITIONS (Details) shares in Thousands, $ in Thousands					3 Months Ended		9 Months Ended
	Jun. 12, 2020 USD ($) country Office shares	Sep. 18, 2019 USD ($)	Jul. 12, 2019 USD ($) Office country	Jan. 02, 2019 USD ($)	Sep. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Acquisitions
Cash			$ 95,034
Deposits			568,161
Goodwill			39,970		$ 793,749		$ 793,749	$ 773,610	$ 773,810
Loans acquired							339,563
Investment securities			$ 39,770
Increase in deposits							1,609,349	$ 277,833
Upstate New York
Acquisitions
Number of new counties where the bank has extended footprints | country			5
Customer Lists
Acquisitions
Cash		$ 500
Intangibles		$ 500
Steuben
Acquisitions
Issuance of common shares (in shares) | shares	1,360
Cash	$ 21,613
Deposits	516,274
Goodwill	20,276
Assets acquired	607,700
Loans acquired	339,500
Investment securities	180,497
Revenues					3,800		4,500
Direct expenses					1,100		1,400
Increase (decrease) in goodwill as a result of fair value adjustment					1,100
Decrease in loans					200
Decrease in premises and equipment					500
Decrease in other assets					1,700
Decrease in accrued interest and other liabilities					1,300
Steuben | New York
Acquisitions
Investment securities	$ 180,500
Steuben | Western New York State
Acquisitions
Number of counties where the bank has facilities | country	4
Number of new counties where the bank has extended footprints | country	2
Number of branch locations | Office	11
Kinderhook Bank Corp
Acquisitions
Cash			$ 93,384
Deposits			568,161
Goodwill			39,970
Assets acquired			642,800
Loans acquired			479,900
Investment securities			39,770
Revenues					3,800		12,700
Direct expenses					$ 2,200		$ 5,800
Increase in deposits			568,200
Increased in accrued interest and fees receivable						$ 300
Increase (decrease) in goodwill as a result of fair value adjustment			40,000			$ 300
Kinderhook Bank Corp | New York
Acquisitions
Cash			93,400
Investment securities			$ 39,800
Kinderhook Bank Corp | Upstate New York
Acquisitions
Number of branch locations | Office			11
Wealth Resources Network, Inc | Customer Lists
Acquisitions
Cash				$ 1,200
Intangibles				$ 1,200

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