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PENSION PLAN AND OTHER BENEFIT PLANS - Additional Information (Details) - USD ($) $ in Thousands |  |  | 12 Months Ended
--- | --- | --- | ---
Jan. 01, 2019 | Oct. 20, 2016 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Hours requirement to be eligible |  | 1000 hours |  |  |
Total unrecognized net loss increased percentage of obligation amortization threshold |  |  | 10.00% |  |
Defined Contribution Profit Sharing, Savings and Investment Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined contribution plan, employer non-discretionary contribution amount |  |  | 3.00% |  |
Contribution cost percentage |  |  | 50.00% |  |
Defined contribution plan, employer matching contribution, percent of employees' gross pay |  |  | 6.00% |  |
Total expense |  |  | $ 1,300 | $ 1,300 | $ 1,200
Chemung common stock included in defined contribution plan assets (in shares) |  |  | 137,698 | 155,696 | 148,716
Defined Contribution Supplemental Executive Retirement Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Total expense |  |  | $ 400 | $ 400 | $ 300
Balance in the plan |  |  | 2,000 | 1,600 |
Pension Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Lump sum settlement payments to employee |  |  | 0 | 0 | 3,300
Settlement charge |  |  | 0 | $ 0 | $ 828
Actuarial Gain (Loss), Increase (Decrease) due to change in discount rate |  |  | $ 3,500 |  |
Percentage of combined loss of expected year end obligations (less than) |  |  | 0.01 |  |
Other comprehensive income, defined benefit plans, increase (decrease) in net unamortized gain loss arising during period net of tax |  |  | $ 800 |  |
Other comprehensive income, defined benefit plans, increase (decrease) in net unamortized gain loss arising during period net of tax, due to variance of actual and expected return on plan assets |  |  | $ (4,300) |  |
Total unrecognized net loss increased (decreased) percentage of plan assets threshold |  |  | 10.00% |  |
Average total life expectancy of participants (years) | 23 years 9 months 10 days |  |  |  |
Pension Plan | Minimum |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Evaluation period of expected long-term return on plan assets |  |  | 3 years |  |
Pension Plan | Maximum |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Evaluation period of expected long-term return on plan assets |  |  | 5 years |  |
Other Postretirement Benefit Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Increase (decrease) in accumulated benefit obligation due to change in discount rate |  |  | $ 10 |  |
Increase (decrease) in accumulated benefit obligation due to discount Rate, claims and date experience |  |  | (65) |  |
Other comprehensive income, defined benefit plans, increase (decrease) in net unamortized gain loss arising during period net of tax |  |  | $ (131) |  |
Contribution cost percentage |  |  | 50.00% |  |
Percentage Increase (Decrease) in unrecognized net loss of the projected benefit obligation |  |  | (0.392) |  |
Estimated employer contributions in next fiscal year |  |  | $ 48 |  |
Average future working lifetime of active participants |  |  | 5 years |  |
Executive Supplemental Pension Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Actuarial Gain (Loss), Increase (Decrease) due to change in discount rate |  |  | $ (69) |  |
Percentage of combined loss of expected year end obligations (less than) |  |  | 0.01 |  |
Actuarial Gain (Loss), Increase (Decrease) due to change in mortality losses |  |  | $ (23) |  |
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% |  |
Average total life expectancy of participants (years) |  |  | 12 years 14 days |  |
Percentage Increase (Decrease) in unrecognized net loss of the projected benefit obligation |  |  | 0.062 |  |
Estimated employer contributions in next fiscal year |  |  | $ 110 |  |
Defined benefit plan, increase(decrease) in total unrecognized net loss |  |  | $ 76 |  |

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