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SECURITIES, OTHER THAN TEMPORARY IMPAIRMENT (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended 3 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Mar. 31, 2014
Dec. 31, 2013
Security
Other than temporary impairment [Abstract]          
Amortized cost of securities sold $ 277,038us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis        
Proceeds from sale of securities 36,258us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt 2,650us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt 26,210us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt    
Other than temporary impairment credit losses recognized in earnings [Roll Forward]          
Beginning balance 1,939us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld 3,506us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld 3,506us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld 1,939us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld  
Amounts related to credit loss for which an other-than-temporary impairment was not previously recognized 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment    
Additions/Subtractions [Abstract]          
Amounts related to securities for which the company intends to sell or that it will be more likely than not that the company will be required to sell prior to recovery of amortized cost basis 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsAdditionalCreditLosses 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsAdditionalCreditLosses 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsAdditionalCreditLosses    
Reductions for increase in cash flows expected to be collected that are recognized over the remaining life of the security 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsCashFlows 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsCashFlows 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsCashFlows    
Reductions for previous credit losses realized on securities sold during the year 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold (1,596)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold    
Reductions for previous credit losses realized in securities liquidated during the year (1,939)chmg_OtherThanTemporaryImpairmentReductionsForPreviousCreditLossesRealizedOnSecuritiesSold 0chmg_OtherThanTemporaryImpairmentReductionsForPreviousCreditLossesRealizedOnSecuritiesSold 0chmg_OtherThanTemporaryImpairmentReductionsForPreviousCreditLossesRealizedOnSecuritiesSold    
Increases to the amount related to the credit loss for which other-than-temporary impairment was previously recognized 0chmg_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsWhenImpairmentRecognizedPreviously 29chmg_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsWhenImpairmentRecognizedPreviously 0chmg_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsWhenImpairmentRecognizedPreviously    
Ending balance 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld 1,939us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld 3,506us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld   1,939us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld
Collateralized Debt Obligations [Member]          
Other than temporary impairment [Abstract]          
Number of securities sold         1chmg_AvailableForSaleSecuritiesNumberOfSecuritiesSold
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember
Amortized cost of securities sold   600us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember
    600us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember
Proceeds from sale of securities         600us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember
Additional credit loss in OTTI recognized         29us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember
Additions/Subtractions [Abstract]          
Income from liquidation of debt securities       $ 500us-gaap_GainLossOnSaleOfDebtInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember