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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) $ / shares in Units, $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 USD ($) Subsidiary Trust Branch County Component Calculation Segment Factor Type $ / shares | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Subsidiary, Sale of Stock [Line Items] |  |  |  |
Number of significant consolidated subsidiaries owned | Subsidiary |  | 2 |  |
Number of unconsolidated subsidiary business trusts owned | Trust |  | 3 |  |
Number of bank branches | Branch |  | 225 |  |
Risks and Uncertainties [Abstract] |  |  |  |
Number of main components of economic risk | Component |  | 3 |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |  |
Period for which a nonaccrual TDR loan remains on nonaccrual status |  | 12 months |  |
Number of days past due for loans to be placed on nonaccrual status |  | 90 days |  |
Allowance for Loan Losses [Abstract] |  |  |  |
Number of broad components used in allowance methodology | Component |  | 2 |  |
Number of calculations used for general loan loss allocations | Calculation |  | 2 |  |
Number of main loan segments | Segment |  | 5 |  |
Period of historical net charge-off data for each loan class used in determining allowance level |  | 36 months |  |
Number of qualitative environmental factors used in qualitative calculation for general loan loss allocations | Factor |  | 8 |  |
Other Real Estate [Abstract] |  |  |  |
Other real estate |  | $ 1,900 | $ 2,000 |
Derivative Financial Instruments and Hedging Activities [Abstract] |  |  |  |
Number of types of derivative financial instruments | Type |  | 3 |  |
Advertising [Abstract] |  |  |  |
Advertising costs |  | $ 5,700 | $ 3,900 | $ 3,600
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Corporate income tax rate |  | 35.00% | 35.00% | 35.00%
Reclassification related to tax effect of Tax Cuts and Jobs Act |  | $ 0 |  |
Minimum [Member] |  |  |  |
Investments in Real Estate Limited Partnerships [Abstract] |  |  |  |
Company's ownership interest in limited partnerships |  | 5.00% |  |
Maximum [Member] |  |  |  |
Investments in Real Estate Limited Partnerships [Abstract] |  |  |  |
Company's ownership interest in limited partnerships |  | 99.99% |  |
Retained Earnings [Member] |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Reclassification related to tax effect of Tax Cuts and Jobs Act |  | $ 645 |  |
Accumulated Other Comprehensive Income/(Loss) [Member] |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Reclassification related to tax effect of Tax Cuts and Jobs Act |  | (645) |  |
ASU 2014-01 [Member] |  |  |  |
Investments in Real Estate Limited Partnerships [Abstract] |  |  |  |
Unfunded commitments |  | 3,200 |  |
Impairment losses related to real estate partnerships |  | 0 |  |
ASU 2016-09 [Member] |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Effect of change on net income from excess tax benefits |  | $ 3,100 |  |
Effect of change on net income from excess tax benefits per diluted common share (in dollars per share) | $ / shares |  | $ 0.06 |  |
ASU 2018-02 [Member] | Retained Earnings [Member] |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Reclassification related to tax effect of Tax Cuts and Jobs Act |  | $ 645 |  |
ASU 2018-02 [Member] | Accumulated Other Comprehensive Income/(Loss) [Member] |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Reclassification related to tax effect of Tax Cuts and Jobs Act |  | $ (645) |  |
Plan [Member] |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Corporate income tax rate | 21.00% |  |  |
Plan [Member] | ASU 2018-02 [Member] |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Corporate income tax rate | 21.00% |  |  |
Upstate New York [Member] |  |  |  |
Subsidiary, Sale of Stock [Line Items] |  |  |  |
Number of counties where the bank has facilities | County |  | 35 |  |
Northeastern Pennsylvania [Member] |  |  |  |
Subsidiary, Sale of Stock [Line Items] |  |  |  |
Number of counties where the bank has facilities | County |  | 6 |  |
Vermont [Member] |  |  |  |
Subsidiary, Sale of Stock [Line Items] |  |  |  |
Number of counties where the bank has facilities | County |  | 12 |  |
Western Massachusetts [Member] |  |  |  |
Subsidiary, Sale of Stock [Line Items] |  |  |  |
Number of counties where the bank has facilities | County |  | 1 |  |
Consumer Installment Loans [Member] |  |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |  |
Number of days past due after which loans are charged off to the extent outstanding principal balance exceeds fair value of collateral/property |  | 90 days |  |
Consumer Mortgage Loans [Member] |  |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |  |
Number of days past due after which loans are charged off to the extent outstanding principal balance exceeds fair value of collateral/property |  | 180 days |  |
Home Equity Loans [Member] |  |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |  |
Number of days past due after which loans are charged off to the extent outstanding principal balance exceeds fair value of collateral/property |  | 180 days |  |
Equipment [Member] | Minimum [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Estimated useful life |  | 3 years |  |
Equipment [Member] | Maximum [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Estimated useful life |  | 10 years |  |
Software [Member] | Minimum [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Estimated useful life |  | 3 years |  |
Software [Member] | Maximum [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Estimated useful life |  | 7 years |  |
Hardware [Member] | Minimum [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Estimated useful life |  | 3 years |  |
Hardware [Member] | Maximum [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Estimated useful life |  | 7 years |  |
Building and Building Improvements [Member] | Minimum [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Estimated useful life |  | 10 years |  |
Building and Building Improvements [Member] | Maximum [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Estimated useful life |  | 40 years |  |
Land Improvements [Member] | Maximum [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Estimated useful life |  | 20 years |  |
BPAS [Member] |  |  |  |
Subsidiary, Sale of Stock [Line Items] |  |  |  |
Number of significant consolidated subsidiaries owned | Subsidiary |  | 5 |  |
Hand Benefits & Trust [Member] |  |  |  |
Subsidiary, Sale of Stock [Line Items] |  |  |  |
Number of significant consolidated subsidiaries owned | Subsidiary |  | 1 |  |
NRS [Member] |  |  |  |
Subsidiary, Sale of Stock [Line Items] |  |  |  |
Number of significant consolidated subsidiaries owned | Subsidiary |  | 1 |  |
Core Deposits [Member] | Minimum [Member] |  |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |  |
Intangible asset useful life (amortization period) |  | 7 years |  |
Core Deposits [Member] | Maximum [Member] |  |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |  |
Intangible asset useful life (amortization period) |  | 20 years |  |
Customer Relationships [Member] | Minimum [Member] |  |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |  |
Intangible asset useful life (amortization period) |  | 7 years |  |
Customer Relationships [Member] | Maximum [Member] |  |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |  |
Intangible asset useful life (amortization period) |  | 20 years |  |
Commercial Portfolio Segment [Member] | Minimum [Member] |  |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |  |
Threshold balance of loans individually evaluated for impairment |  | $ 500 |  |

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