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