PENSION PLAN AND OTHER BENEFIT PLANS - Additional Information (Details) - USD ($) $ in Thousands			12 Months Ended
	Jan. 01, 2019	Oct. 20, 2016	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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]
Total expense			$ 1,300	$ 1,200	$ 1,160
Chemung common stock included in defined contribution plan assets (in shares)			155,696	148,716	155,336
Defined Contribution Supplemental Executive Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Total expense			$ 400	$ 300	$ 300
Balance in the plan			1,600	1,200
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Lump sum settlement payments to employee			0	3,288
Settlement charge			0	$ 828	$ 0
Other comprehensive income, defined benefit plans, increase (decrease) in net unamortized gain loss arising during period net of tax			$ 300
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	24 years 6 months 16 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			$ 74
Average future working lifetime of active participants			5 years
Executive Supplemental 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%
Average total life expectancy of participants	12 years 8 months 24 days
Defined benefit plan, increase(decrease) in total unrecognized net loss			$ 109