XML 52 R40.htm IDEA: XBRL DOCUMENT
v3.10.0.1

SECURITIES Securities - Narrative (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Gain (Loss) on Investments [Line Items] |  |  |  |
Change in fair value of equity securities | $ 2,141 | $ 32 | $ 2,165 | $ 96
Visa Class B |  |  |  |
Gain (Loss) on Investments [Line Items] |  |  |  |
Change in fair value of equity securities | $ 2,100 |  | $ 2,100 |

X
- Definition Equity Securities, FV-NI, Gain (Loss) + References No definition available. + Details
Name: | chmg_EquitySecuritiesFVNIGainLoss
Namespace Prefix: | chmg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_ScheduleOfGainLossOnInvestmentsIncludingMarketableSecuritiesAndInvestmentsHeldAtCostIncomeStatementReportedAmountsSummaryLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=chmg_VisaClassBMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |