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CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (USD $)
In Thousands, unless otherwise specified
Common Stock [Member]
Additional Paid-in Capital [Member]
Retained Earnings [Member]
Treasury Stock [Member]
Accumulated Other Comprehensive Income (Loss) [Member]
Total
Balances at Dec. 31, 2011 $ 53us-gaap_StockholdersEquity
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= us-gaap_CommonStockMember
$ 45,582us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
$ 100,629us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
$ (18,894)us-gaap_StockholdersEquity
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= us-gaap_TreasuryStockMember
$ (1,441)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
$ 125,929us-gaap_StockholdersEquity
Increase (Decrease) in Shareholders' Equity [Roll Forward]            
Net income 0us-gaap_NetIncomeLoss
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= us-gaap_CommonStockMember
0us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
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11,022us-gaap_NetIncomeLoss
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= us-gaap_RetainedEarningsMember
0us-gaap_NetIncomeLoss
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= us-gaap_TreasuryStockMember
0us-gaap_NetIncomeLoss
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= us-gaap_AccumulatedOtherComprehensiveIncomeMember
11,022us-gaap_NetIncomeLoss
Other comprehensive income (loss) 0us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
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= us-gaap_CommonStockMember
0us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
0us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
(1,366)us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
(1,366)us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
Restricted stock awards 0us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRestrictedStockUnitsRequisiteServicePeriodRecognition
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
80us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRestrictedStockUnitsRequisiteServicePeriodRecognition
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRestrictedStockUnitsRequisiteServicePeriodRecognition
/ us-gaap_StatementEquityComponentsAxis
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0us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRestrictedStockUnitsRequisiteServicePeriodRecognition
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0us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRestrictedStockUnitsRequisiteServicePeriodRecognition
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
80us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRestrictedStockUnitsRequisiteServicePeriodRecognition
Distribution of shares of treasury stock granted for directors' deferred compensation plan 0chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationValue
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= us-gaap_CommonStockMember
(82)chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
83chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
0chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
1chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationValue
Distribution of shares of treasury stock granted for employee restricted stock awards, net 0chmg_DistributionShareOfTreasuryStockForEmployeeRestrictedStockWarrants
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(274)chmg_DistributionShareOfTreasuryStockForEmployeeRestrictedStockWarrants
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0chmg_DistributionShareOfTreasuryStockForEmployeeRestrictedStockWarrants
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274chmg_DistributionShareOfTreasuryStockForEmployeeRestrictedStockWarrants
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= us-gaap_TreasuryStockMember
0chmg_DistributionShareOfTreasuryStockForEmployeeRestrictedStockWarrants
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0chmg_DistributionShareOfTreasuryStockForEmployeeRestrictedStockWarrants
Restricted stock units for directors' deferred compensation plan 0chmg_RestrictedStockUnitsForDirectorsDeferrredCompensationPlan
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87chmg_RestrictedStockUnitsForDirectorsDeferrredCompensationPlan
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0chmg_RestrictedStockUnitsForDirectorsDeferrredCompensationPlan
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0chmg_RestrictedStockUnitsForDirectorsDeferrredCompensationPlan
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= us-gaap_TreasuryStockMember
0chmg_RestrictedStockUnitsForDirectorsDeferrredCompensationPlan
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
87chmg_RestrictedStockUnitsForDirectorsDeferrredCompensationPlan
Cash dividends declared 0us-gaap_DividendsCommonStockCash
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0us-gaap_DividendsCommonStockCash
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(4,573)us-gaap_DividendsCommonStockCash
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0us-gaap_DividendsCommonStockCash
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0us-gaap_DividendsCommonStockCash
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(4,573)us-gaap_DividendsCommonStockCash
Distribution of shares of treasury stock for directors' compensation 0chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationValue
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(28)chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationValue
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0chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationValue
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261chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationValue
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= us-gaap_TreasuryStockMember
0chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
233chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationValue
Distribution of shares of treasury stock for employee compensation 0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
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= us-gaap_CommonStockMember
(8)us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
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0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
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88us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
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0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
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= us-gaap_AccumulatedOtherComprehensiveIncomeMember
80us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
Sale of shares of treasury stock 0us-gaap_StockIssuedDuringPeriodValueTreasuryStockReissued
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0us-gaap_StockIssuedDuringPeriodValueTreasuryStockReissued
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0us-gaap_StockIssuedDuringPeriodValueTreasuryStockReissued
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= us-gaap_RetainedEarningsMember
258us-gaap_StockIssuedDuringPeriodValueTreasuryStockReissued
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= us-gaap_TreasuryStockMember
0us-gaap_StockIssuedDuringPeriodValueTreasuryStockReissued
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= us-gaap_AccumulatedOtherComprehensiveIncomeMember
258us-gaap_StockIssuedDuringPeriodValueTreasuryStockReissued
Purchase of shares of treasury stock 0us-gaap_TreasuryStockValueAcquiredCostMethod
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0us-gaap_TreasuryStockValueAcquiredCostMethod
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0us-gaap_TreasuryStockValueAcquiredCostMethod
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(636)us-gaap_TreasuryStockValueAcquiredCostMethod
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0us-gaap_TreasuryStockValueAcquiredCostMethod
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(636)us-gaap_TreasuryStockValueAcquiredCostMethod
Balances at Dec. 31, 2012 53us-gaap_StockholdersEquity
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45,357us-gaap_StockholdersEquity
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107,078us-gaap_StockholdersEquity
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(18,566)us-gaap_StockholdersEquity
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(2,807)us-gaap_StockholdersEquity
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= us-gaap_AccumulatedOtherComprehensiveIncomeMember
131,115us-gaap_StockholdersEquity
Increase (Decrease) in Shareholders' Equity [Roll Forward]            
Net income 0us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
0us-gaap_NetIncomeLoss
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8,731us-gaap_NetIncomeLoss
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= us-gaap_RetainedEarningsMember
0us-gaap_NetIncomeLoss
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0us-gaap_NetIncomeLoss
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8,731us-gaap_NetIncomeLoss
Other comprehensive income (loss) 0us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
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0us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
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0us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
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0us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
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2,962us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
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2,962us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
Restricted stock awards 0us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRestrictedStockUnitsRequisiteServicePeriodRecognition
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= us-gaap_CommonStockMember
131us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRestrictedStockUnitsRequisiteServicePeriodRecognition
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0us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRestrictedStockUnitsRequisiteServicePeriodRecognition
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0us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRestrictedStockUnitsRequisiteServicePeriodRecognition
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0us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRestrictedStockUnitsRequisiteServicePeriodRecognition
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= us-gaap_AccumulatedOtherComprehensiveIncomeMember
131us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRestrictedStockUnitsRequisiteServicePeriodRecognition
Distribution of shares of treasury stock granted for directors' deferred compensation plan 0chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationValue
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= us-gaap_CommonStockMember
(75)chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationValue
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= us-gaap_AdditionalPaidInCapitalMember
0chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationValue
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86chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationValue
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0chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
11chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationValue
Distribution of shares of treasury stock granted for employee restricted stock awards, net 0chmg_DistributionShareOfTreasuryStockForEmployeeRestrictedStockWarrants
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(206)chmg_DistributionShareOfTreasuryStockForEmployeeRestrictedStockWarrants
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0chmg_DistributionShareOfTreasuryStockForEmployeeRestrictedStockWarrants
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206chmg_DistributionShareOfTreasuryStockForEmployeeRestrictedStockWarrants
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= us-gaap_TreasuryStockMember
0chmg_DistributionShareOfTreasuryStockForEmployeeRestrictedStockWarrants
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0chmg_DistributionShareOfTreasuryStockForEmployeeRestrictedStockWarrants
Restricted stock units for directors' deferred compensation plan 0chmg_RestrictedStockUnitsForDirectorsDeferrredCompensationPlan
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99chmg_RestrictedStockUnitsForDirectorsDeferrredCompensationPlan
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0chmg_RestrictedStockUnitsForDirectorsDeferrredCompensationPlan
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0chmg_RestrictedStockUnitsForDirectorsDeferrredCompensationPlan
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0chmg_RestrictedStockUnitsForDirectorsDeferrredCompensationPlan
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99chmg_RestrictedStockUnitsForDirectorsDeferrredCompensationPlan
Cash dividends declared 0us-gaap_DividendsCommonStockCash
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0us-gaap_DividendsCommonStockCash
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(4,778)us-gaap_DividendsCommonStockCash
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0us-gaap_DividendsCommonStockCash
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0us-gaap_DividendsCommonStockCash
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(4,778)us-gaap_DividendsCommonStockCash
Distribution of shares of treasury stock for directors' compensation 0chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationValue
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14chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationValue
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0chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationValue
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203chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationValue
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0chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationValue
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217chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationValue
Distribution of shares of treasury stock for employee compensation 0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
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7us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
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105us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
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0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
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112us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
Forfeit of restricted stock awards 0us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures
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61us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures
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0us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures
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(61)us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures
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0us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures
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0us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures
Sale of shares of treasury stock 0us-gaap_StockIssuedDuringPeriodValueTreasuryStockReissued
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11us-gaap_StockIssuedDuringPeriodValueTreasuryStockReissued
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0us-gaap_StockIssuedDuringPeriodValueTreasuryStockReissued
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60us-gaap_StockIssuedDuringPeriodValueTreasuryStockReissued
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0us-gaap_StockIssuedDuringPeriodValueTreasuryStockReissued
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71us-gaap_StockIssuedDuringPeriodValueTreasuryStockReissued
Purchase of shares of treasury stock 0us-gaap_TreasuryStockValueAcquiredCostMethod
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0us-gaap_TreasuryStockValueAcquiredCostMethod
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0us-gaap_TreasuryStockValueAcquiredCostMethod
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0us-gaap_TreasuryStockValueAcquiredCostMethod
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(93)us-gaap_TreasuryStockValueAcquiredCostMethod
Balances at Dec. 31, 2013 53us-gaap_StockholdersEquity
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45,399us-gaap_StockholdersEquity
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111,031us-gaap_StockholdersEquity
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(18,060)us-gaap_StockholdersEquity
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155us-gaap_StockholdersEquity
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138,578us-gaap_StockholdersEquity
Increase (Decrease) in Shareholders' Equity [Roll Forward]            
Net income 0us-gaap_NetIncomeLoss
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0us-gaap_NetIncomeLoss
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0us-gaap_NetIncomeLoss
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Other comprehensive income (loss) 0us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
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(8,940)us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
Restricted stock awards 0us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRestrictedStockUnitsRequisiteServicePeriodRecognition
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0us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRestrictedStockUnitsRequisiteServicePeriodRecognition
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151us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRestrictedStockUnitsRequisiteServicePeriodRecognition
Distribution of shares of treasury stock granted for directors' deferred compensation plan 0chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationValue
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(85)chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationValue
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0chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationValue
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88chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationValue
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0chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationValue
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3chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationValue
Distribution of shares of treasury stock granted for employee restricted stock awards, net 0chmg_DistributionShareOfTreasuryStockForEmployeeRestrictedStockWarrants
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(288)chmg_DistributionShareOfTreasuryStockForEmployeeRestrictedStockWarrants
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0chmg_DistributionShareOfTreasuryStockForEmployeeRestrictedStockWarrants
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
288chmg_DistributionShareOfTreasuryStockForEmployeeRestrictedStockWarrants
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
0chmg_DistributionShareOfTreasuryStockForEmployeeRestrictedStockWarrants
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0chmg_DistributionShareOfTreasuryStockForEmployeeRestrictedStockWarrants
Restricted stock units for directors' deferred compensation plan 0chmg_RestrictedStockUnitsForDirectorsDeferrredCompensationPlan
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
94chmg_RestrictedStockUnitsForDirectorsDeferrredCompensationPlan
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0chmg_RestrictedStockUnitsForDirectorsDeferrredCompensationPlan
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
0chmg_RestrictedStockUnitsForDirectorsDeferrredCompensationPlan
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
0chmg_RestrictedStockUnitsForDirectorsDeferrredCompensationPlan
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
94chmg_RestrictedStockUnitsForDirectorsDeferrredCompensationPlan
Cash dividends declared 0us-gaap_DividendsCommonStockCash
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
0us-gaap_DividendsCommonStockCash
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
(4,805)us-gaap_DividendsCommonStockCash
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
0us-gaap_DividendsCommonStockCash
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
0us-gaap_DividendsCommonStockCash
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
(4,805)us-gaap_DividendsCommonStockCash
Distribution of shares of treasury stock for directors' compensation 0chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
59chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
214chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
0chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
273chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationValue
Distribution of shares of treasury stock for employee compensation 0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
25us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
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92us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
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0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
117us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
Balances at Dec. 31, 2014 $ 53us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
$ 45,355us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
$ 114,383us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
$ (17,378)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
$ (8,785)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
$ 133,628us-gaap_StockholdersEquity