SECURITIES - Sales and Calls (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Proceeds	$ 0	$ 545	$ 0	$ 1,620
Gross gains	0	0	0	12
Tax expense	$ 0	$ 0	$ 0	$ 4

This item represents the tax expense on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

No definition available.

Name:	chmg_TaxExpenseFromSaleOfAvailableForSaleSecuritiesDebt
Namespace Prefix:	chmg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration