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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details) - USD ($) $ in Thousands | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Property, Plant and Equipment [Line Items] |  |  |  |  |
Adjustments due to new accounting pronouncements |  | $ 199,699 | $ 182,627 | $ 165,029 | $ 149,813
Period of delinquency after which a loan is placed on non-accrual status (days) |  | 90 days |  |  |
Number of consecutive months for which loans remain current before non-accrual loans are returned to accrual status |  | 6 months |  |  |
Rolling historical period of net losses used in evaluating general component of valuation allowance for loans not impaired but classified as substandard and special mention (years) |  | 5 years |  |  |
Number of most recent years of actual loss history used in determining historical loss experience for all other unclassified loans (years) |  | 2 years |  |  |
Assets under management |  | $ 2,091,000 | 1,915,000 |  |
Buildings | Minimum |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Estimated useful lives (years) |  | 15 years |  |  |
Buildings | Maximum |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Estimated useful lives (years) |  | 50 years |  |  |
Equipment and Furniture | Minimum |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Estimated useful lives (years) |  | 3 years |  |  |
Equipment and Furniture | Maximum |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Estimated useful lives (years) |  | 10 years |  |  |
Chemung Financial Corporation |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Assets under management |  | $ 305,500 | 289,700 |  |
Retained Earnings |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Adjustments due to new accounting pronouncements |  | 168,006 | 153,701 | 143,129 | 128,453
Accumulated Other Comprehensive Income (Loss) |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Adjustments due to new accounting pronouncements |  | $ 2,401 | $ (5,799) | $ (11,411) | (10,340)
Accumulated Other Comprehensive Income (Loss) | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Adjustments due to new accounting pronouncements |  |  |  |  | (202)
Accounting Standards Update 2016-01 | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Adjustments due to new accounting pronouncements | [1] |  |  |  | (162)
Accounting Standards Update 2016-01 | Retained Earnings | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Adjustments due to new accounting pronouncements | [1] |  |  |  | 40
Accounting Standards Update 2016-01 | Accumulated Other Comprehensive Income (Loss) | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Adjustments due to new accounting pronouncements | [1] |  |  |  | $ (202)
[1] | Due to implementation of ASC 2016-01. See "Adoption of New Accounting Standards" discussion in Note 1.

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