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