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This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. true2Net Income90false 0truefalsetruefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse17false USDtruefalse$FROM_Apr01_2013_TO_Jun30_2013_dei_LegalEntityAxis_WesternMassachusettsElectricCompanyMemberhttp://www.sec.gov/CIK0000072741duration2013-04-01T00:00:002013-06-30T00:00:00falsefalseWestern Massachusetts Electric Company [Member]dei_LegalEntityAxisxbrldihttp://xbrl.org/2006/xbrldinu_WesternMassachusettsElectricCompanyMemberdei_LegalEntityAxisexplicitMemberUSDStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$nanafalse0Western Massachusetts Electric Company [Member]91false 4us-gaap_Revenuesus-gaap_truecreditdurationfalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse115015000115015USD$falsefalsefalse2truefalsefalse106836000106836USD$falsefalsefalse3truefalsefalse239968000239968USD$falsefalsefalse4truefalsefalse220861000220861USD$falsefalsefalsexbrli:monetaryItemTypemonetaryAggregate revenue recognized during the period (derived from goods sold, services rendered, insurance premiums, or other activities that constitute an entity's earning process). 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Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 false2Depreciation96false 5us-gaap_AmortizationOfRegulatoryAssetus-gaap_truedebitdurationfalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse685000685USD$falsefalsefalse2truefalsefalse-44000-44USD$falsefalsefalse3truefalsefalse814000814USD$falsefalsefalse4truefalsefalse-387000-387USD$falsefalsefalsexbrli:monetaryItemTypemonetaryThe expense charged against earnings during the period to allocate the capitalized costs of regulatory assets over the periods expected to benefit from such costs.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. 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Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.3) -URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688 false2Amortization of Regulatory Assets/(Liabilities), Net97false 5us-gaap_OtherDepreciationAndAmortizationus-gaap_truedebitdurationfalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse30910003091USD$falsefalsefalse2truefalsefalse43580004358USD$falsefalsefalse3truefalsefalse77800007780USD$falsefalsefalse4truefalsefalse87760008776USD$falsefalsefalsexbrli:monetaryItemTypemonetaryThe other noncash expense, not otherwise specified in the taxonomy, charged against earnings in the period to allocate the cost of tangible and intangible assets over their remaining economic lives.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. 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