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PENSION PLAN AND OTHER BENEFIT PLANS - Additional Information (Details) - USD ($) $ in Thousands |  |  | 10 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jan. 01, 2018 | Jan. 01, 2016 | Oct. 19, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Hours requirement to be eligible |  |  | 1000 hours |  |  |
Defined Contribution Profit Sharing, Savings and Investment Plan |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Defined contribution plan, employer non-discretionary contribution amount |  |  |  | 3.00% |  |
Defined contribution plan, employer matching contribution, percent of employees' gross pay |  |  |  | 6.00% |  |
Chemung common stock included in defined contribution plan assets (in shares) |  |  |  | 155,336 | 174,957 | 169,398
Total expense |  |  |  | $ 1,160 | $ 609 | $ 639
Defined Contribution Supplemental Executive Retirement Plan |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Balance in the plan |  |  |  | 872 | 1,043 |
Total expense |  |  |  | $ 279 | $ 262 | $ 231
Pension Plan |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Total unrecognized net loss increased percentage of obligation amortization threshold |  |  |  | 10.00% |  |
Total unrecognized net loss increased(decreased) percentage of plan assets threshold |  |  |  | 10.00% |  |
Other comprehensive income, defined benefit plans, increase (decrease) in net unamortized gain loss arising during period net of tax |  |  |  | $ 1,400 |  |
Average total life expectancy of participants |  | 27 years 9 months 15 days |  |  |  |
Other Postretirement Benefit Plan |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Other comprehensive income, defined benefit plans, increase (decrease) in net unamortized gain loss arising during period net of tax |  |  |  | $ 45 |  |
Average future working lifetime of active participants |  |  |  | 5 years |  |
Executive Supplemental Pension Plan |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Defined benefit plan, increase(decrease) in total unrecognized net loss |  |  |  | $ 64 |  |
Total unrecognized net loss increased percentage of obligation amortization threshold |  |  |  | 10.00% |  |
Total unrecognized net loss increased(decreased) percentage of plan assets threshold |  |  |  | 10.00% |  |
Scenario, Forecast | Executive Supplemental Pension Plan |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Average total life expectancy of participants | 14 years 2 months 13 days |  |  |  |  |

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