2.4.0.8400095 - Disclosure - DERIVATIVE INSTURMENTS (Details)truefalsefalse1false USDfalsefalse$FROM_Apr01_2013_TO_Jun30_2013http://www.sec.gov/CIK0000072741duration2013-04-01T00:00:002013-06-30T00:00:00USDStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$2false USDfalsefalse$FROM_Apr01_2012_TO_Jun30_2012http://www.sec.gov/CIK0000072741duration2012-04-01T00:00:002012-06-30T00:00:00USDStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$3false USDfalsefalse$FROM_Jan01_2013_TO_Jun30_2013http://www.sec.gov/CIK0000072741duration2013-01-01T00:00:002013-06-30T00:00:00USDStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$4false USDfalsefalse$FROM_Jan01_2012_TO_Jun30_2012http://www.sec.gov/CIK0000072741duration2012-01-01T00:00:002012-06-30T00:00:00USDStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$5false USDfalsefalse$AS_OF_Dec31_2012http://www.sec.gov/CIK0000072741instant2012-12-31T00:00:000001-01-01T00:00:00USDStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$1true 4us-gaap_DerivativeInstrumentDetailAbstractus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0Derivative Instrument Detail Abstract2false 5us-gaap_DerivativeLiabilitiesNoncurrentus-gaap_truecreditinstantfalsefalsefalsefalsefalsefalsefalsetruenegatedLabel1truefalsefalse-788929000-788929000USD$falsetruefalse2falsefalsefalse00falsefalsefalse3truefalsefalse-788929000-788929000USD$falsetruefalse4falsefalsefalse00falsefalsefalse5truefalsefalse-882654000-882654000USD$falsetruefalsexbrli:monetaryItemTypemonetaryFair values as of the balance sheet date of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, and which are expected to be extinguished or otherwise disposed of after one year or beyond the normal operating cycle, if longer, net of the effects of master netting arrangements.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Research Bulletin (ARB) -Number 43 -Section A -Paragraph 7 -Chapter 3 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. 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This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. false2Derivative Assets Noncurrent4false 5nu_CurrentDerivativeAssetsConsolidatednu_falsedebitinstantfalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse91000009100000USD$falsefalsefalse2falsefalsefalse00falsefalsefalse3truefalsefalse91000009100000USD$falsefalsefalse4falsefalsefalse00falsefalsefalse5truefalsefalse1140000011400000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryRepresents consolidated current derivative assets.No definition available.false2Current Derivative Assets Consolidated5false 5us-gaap_DerivativeCollateralRightToReclaimCashus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse21000002100000USD$falsefalsefalse2falsefalsefalse00falsefalsefalse3truefalsefalse21000002100000USD$falsefalsefalse4falsefalsefalse00falsefalsefalse5truefalsefalse41000004100000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryThe amount of the right to reclaim cash collateral under master netting arrangements that have not been offset against net derivative instrument positions.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7476318&loc=d3e41678-113959 false2DerivativeCollateralRightToReclaimCash6false 5nu_ConsolidatedCurrentDerivativeLiabilitiesnu_falsecreditinstantfalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse109000000109000000USD$falsefalsefalse2falsefalsefalse00falsefalsefalse3truefalsefalse109000000109000000USD$falsefalsefalse4falsefalsefalse00falsefalsefalse5truefalsefalse117200000117200000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryCurrent derivative liabilitiesNo definition available.false2Consolidated Current Derivative Liabilities7true 4us-gaap_OtherDerivativesNotDesignatedAsHedgingInstrumentsAtFairValueNetTotalAbstractus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract8false 5us-gaap_DescriptionOfDerivativeActivityVolumeus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00As of June 30, 2013 and December 31, 2012, NU had approximately 2 thousand MWh and 24 thousand MWh, respectively, of supply volumes remaining in its unregulated wholesale portfolio when expected sales are compared with supply contracts.falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringInformation that would enable users to understand the volume of the entity's derivative activity.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5579245-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5580258-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 44 -Subparagraph 2 -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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This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. false2Derivative Net Liability Position Aggregate Fair Value14false 5us-gaap_CollateralAlreadyPostedAggregateFairValueus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5truefalsefalse00USD$falsefalsefalsexbrli:monetaryItemTypemonetaryThe aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 44D -Subparagraph c -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 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624258-113959 false2Collateral Already Posted Aggregate Fair Value15false 5us-gaap_AdditionalCollateralAggregateFairValueus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse1140000011400000USD$falsefalsefalse2falsefalsefalse00falsefalsefalse3truefalsefalse1140000011400000USD$falsefalsefalse4falsefalsefalse00falsefalsefalse5truefalsefalse1740000017400000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryThe aggregate fair value of additional assets that would be required to be posted as collateral for derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624258-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 44D -Subparagraph c -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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sheet date of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, and which are expected to be extinguished or otherwise disposed of within a year or the normal operating cycle, if longer, net of the effects of master netting arrangements.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Current Liabilities -URI http://asc.fasb.org/extlink&oid=6509677 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Research Bulletin (ARB) -Number 43 -Section A -Paragraph 7 -Chapter 3 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13433-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13495-108611 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FIN39-1 -Paragraph 10A, 10B -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 4, 17 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. false2Derivative Liabilities - Current33false 5us-gaap_DerivativeLiabilitiesNoncurrentus-gaap_truecreditinstantfalsefalsefalsefalsefalsefalsefalsetruenegatedLabel1truefalsefalse-777144000-777144000USD$falsefalsefalse2falsefalsefalse00falsefalsefalse3truefalsefalse-777144000-777144000USD$falsefalsefalse4falsefalsefalse00falsefalsefalse5truefalsefalse-865571000-865571000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryFair values as of the balance sheet date of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, and which are expected to be extinguished or otherwise disposed of after one year or beyond the normal operating cycle, if longer, net of the effects of master netting arrangements.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Research Bulletin (ARB) -Number 43 -Section A -Paragraph 7 -Chapter 3 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13433-108611 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 107 -Paragraph 10 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13495-108611 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FIN39-1 -Paragraph 10A, 10B -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 4, 17 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. false2Derivative Liabilities, Noncurrent34false 5us-gaap_DerivativeAssetsNoncurrentus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse8930900089309000USD$falsefalsefalse2falsefalsefalse00falsefalsefalse3truefalsefalse8930900089309000USD$falsefalsefalse4falsefalsefalse00falsefalsefalse5truefalsefalse9061200090612000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryFair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments which are expected to exist longer than one year or beyond the normal operating cycle, if longer, net of the effects of master netting arrangements.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13433-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 107 -Paragraph 10 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13495-108611 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Research Bulletin (ARB) -Number 43 -Section A -Paragraph 4 -Chapter 3 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FIN39-1 -Paragraph 10A, 10B -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 4, 17 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. false2Derivative Assets Noncurrent35true 4us-gaap_OtherDerivativesNotDesignatedAsHedgingInstrumentsAtFairValueNetTotalAbstractus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract36false 5us-gaap_DescriptionOfDerivativeActivityVolumeus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00Commodity Supply and Price Risk Management: As required by regulation, CL&P has capacity-related contracts with generation facilities. These contracts and similar UI contracts have an expected capacity of 787 MW. CL&P has a sharing agreement with UI, with 80 percent of each contract allocated to CL&P and 20 percent allocated to UI. The capacity contracts extend through 2026 and obligate the utilities to make or receive payments on a monthly basis to or from the generation facilities based on the difference between a set capacity price and the forward capacity market price received in the ISO-NE capacity markets. In addition, CL&P has a contract to purchase 0.1 million MWh of energy per year through 2020.falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringInformation that would enable users to understand the volume of the entity's derivative activity.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5579245-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5580258-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 44 -Subparagraph 2 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. false0Description of Derivative Activity Volume37false 0truefalsetruefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalse12false USDtruefalse$AS_OF_Jun30_2013_dei_LegalEntityAxis_TheConnecticutLightAndPowerCompanyMember_us-gaap_RangeAxis_MinimumMemberhttp://www.sec.gov/CIK0000072741instant2013-06-30T00:00:000001-01-01T00:00:00falsefalseThe Connecticut Light And Power Company [Member]dei_LegalEntityAxisxbrldihttp://xbrl.org/2006/xbrldinu_TheConnecticutLightAndPowerCompanyMemberdei_LegalEntityAxisexplicitMemberfalsefalseMinimumMemberus-gaap_RangeAxisxbrldihttp://xbrl.org/2006/xbrldius-gaap_MinimumMemberus-gaap_RangeAxisexplicitMemberUSDPerMegaWattHourDividehttp://www.xbrl.org/2003/iso4217USDiso4217http://www.nu.com/20130630MWhnu0USDPerKilowattMonthDividehttp://www.xbrl.org/2003/iso4217USDiso4217http://www.nu.com/20130630KWmonu0USDUSD$nanafalse0The Connecticut Light And Power Company [Member] | MinimumMember38true 4us-gaap_FairValueInputsQuantitativeInformationAbstractus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0FairValueInputsQuantitativeInformation[Abstract]39false 5nu_Fairvalueinputscapacitypricesnu_falsenainstantfalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse1.41.4falsefalsefalse2falsefalsefalse00falsefalsefalse3truefalsefalse1.41.4falsefalsefalse4falsefalsefalse00falsefalsefalse5truefalsefalse1.41.4falsefalsefalseus-types:perUnitItemTypedecimalRepresents capacity prices used in derivative valuations for periods not yet observable.No definition available.false0FairValueInputsCapacityPrices40false 5nu_Fairvalueinputsenergypricesnu_falsenainstantfalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse5353falsefalsefalse2falsefalsefalse00falsefalsefalse3truefalsefalse5353falsefalsefalse4falsefalsefalse00falsefalsefalse5truefalsefalse5151falsefalsefalseus-types:perUnitItemTypedecimalRepresents the prices used for energy in Level 3 contracts for unobservable periods.No definition available.false0FairValueInputsEnergyPrices41false 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[Member]dei_LegalEntityAxisxbrldihttp://xbrl.org/2006/xbrldinu_TheConnecticutLightAndPowerCompanyMemberdei_LegalEntityAxisexplicitMemberfalsefalseMaximumMemberus-gaap_RangeAxisxbrldihttp://xbrl.org/2006/xbrldius-gaap_MaximumMemberus-gaap_RangeAxisexplicitMemberUSDPerKilowattMonthDividehttp://www.xbrl.org/2003/iso4217USDiso4217http://www.nu.com/20130630KWmonu0USDPerMegaWattHourDividehttp://www.xbrl.org/2003/iso4217USDiso4217http://www.nu.com/20130630MWhnu0USDUSD$nanafalse0The Connecticut Light And Power Company [Member] | MaximumMember43true 4us-gaap_FairValueInputsQuantitativeInformationAbstractus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0FairValueInputsQuantitativeInformation[Abstract]44false 5nu_Fairvalueinputscapacitypricesnu_falsenainstantfalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse9.519.51falsefalsefalse2falsefalsefalse00falsefalsefalse3truefalsefalse9.519.51falsefalsefalse4falsefalsefalse00falsefalsefalse5truefalsefalse9.839.83falsefalsefalseus-types:perUnitItemTypedecimalRepresents capacity prices used in derivative valuations for periods not yet observable.No definition available.false0FairValueInputsCapacityPrices45false 5nu_Fairvalueinputsenergypricesnu_falsenainstantfalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse5656falsefalsefalse2falsefalsefalse00falsefalsefalse3truefalsefalse5656falsefalsefalse4falsefalsefalse00falsefalsefalse5truefalsefalse5555falsefalsefalseus-types:perUnitItemTypedecimalRepresents the prices used for energy in Level 3 contracts for unobservable periods.No definition available.false0FairValueInputsEnergyPrices46false 5nu_Fairvalueinputsforwardreservenu_falsenainstantfalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse33falsefalsefalse2falsefalsefalse00falsefalsefalse3truefalsefalse33falsefalsefalse4falsefalsefalse00falsefalsefalse5truefalsefalse0.90.9falsefalsefalseus-types:perUnitItemTypedecimalRepresents forward reserve market prices used in 2 of the CfDsNo definition available.false0FairValueInputsForwardReserve47false 0truefalsetruefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalse16false truefalseFROM_Jan01_2013_TO_Jun30_2013_dei_LegalEntityAxis_NstarElectricCompanyMemberhttp://www.sec.gov/CIK0000072741duration2013-01-01T00:00:002013-06-30T00:00:00falsefalseNSTAR Electric Company [Member]dei_LegalEntityAxisxbrldihttp://xbrl.org/2006/xbrldinu_NstarElectricCompanyMemberdei_LegalEntityAxisexplicitMembernanafalse0NSTAR Electric Company [Member]48true 4us-gaap_OtherDerivativesNotDesignatedAsHedgingInstrumentsAtFairValueNetTotalAbstractus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract49false 5us-gaap_DescriptionOfDerivativeActivityVolumeus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00NSTAR Electric has a renewable energy contract to purchase 0.1 million MWh of energy per year through 2018. NSTAR Electric also has a capacity related contract for up to 35 MW per year that extends through 2019. PSNH has electricity procurement contracts to purchase 0.3 million MWh of energy through November 2013.falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringInformation that would enable users to understand the volume of the entity's derivative activity.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5579245-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5580258-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 44 -Subparagraph 2 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. false0Description of Derivative Activity Volume50false 0truefalsetruefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalse17false USDtruefalse$AS_OF_Jun30_2013_dei_LegalEntityAxis_NstarElectricCompanyMember_us-gaap_RangeAxis_MinimumMemberhttp://www.sec.gov/CIK0000072741instant2013-06-30T00:00:000001-01-01T00:00:00falsefalseNSTAR Electric Company [Member]dei_LegalEntityAxisxbrldihttp://xbrl.org/2006/xbrldinu_NstarElectricCompanyMemberdei_LegalEntityAxisexplicitMemberfalsefalseMinimumMemberus-gaap_RangeAxisxbrldihttp://xbrl.org/2006/xbrldius-gaap_MinimumMemberus-gaap_RangeAxisexplicitMemberUSDPerKilowattMonthDividehttp://www.xbrl.org/2003/iso4217USDiso4217http://www.nu.com/20130630KWmonu0USDPerRecDividehttp://www.xbrl.org/2003/iso4217USDiso4217http://www.nu.com/20130630RECnu0USDUSD$nanafalse0NSTAR Electric Company [Member] | MinimumMember51true 4us-gaap_FairValueInputsQuantitativeInformationAbstractus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0FairValueInputsQuantitativeInformation[Abstract]52false 5nu_Fairvalueinputscapacitypricesnu_falsenainstantfalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse1.41.4falsefalsefalse2falsefalsefalse00falsefalsefalse3truefalsefalse1.41.4falsefalsefalse4falsefalsefalse00falsefalsefalse5truefalsefalse1.41.4falsefalsefalseus-types:perUnitItemTypedecimalRepresents capacity prices used in derivative valuations for periods not yet observable.No definition available.false0FairValueInputsCapacityPrices53false 5nu_Fairvalueinputsrenewableenergycreditpricesnu_falsenainstantfalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse2525falsefalsefalse2falsefalsefalse00falsefalsefalse3truefalsefalse2525falsefalsefalse4falsefalsefalse00falsefalsefalse5truefalsefalse2525falsefalsefalseus-types:perUnitItemTypedecimalRepresents the price per renewable energy credit.No definition available.false0FairValueInputsRenewableEnergyCreditPrices54false 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[Member]dei_LegalEntityAxisxbrldihttp://xbrl.org/2006/xbrldinu_PublicServiceCompanyOfNewHampshireMemberdei_LegalEntityAxisexplicitMemberUSDStandardhttp://www.xbrl.org/2003/iso4217USDiso42170$nanafalse0Public Service Company Of New Hampshire [Member]59true 4us-gaap_OtherDerivativesNotDesignatedAsHedgingInstrumentsAtFairValueNetTotalAbstractus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract60false 5us-gaap_DescriptionOfDerivativeActivityVolumeus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00PSNH has electricity procurement contracts to purchase 0.3 million MWh of energy through November 2013.falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringInformation that would enable users to understand the volume of the entity's derivative activity.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5579245-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5580258-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 44 -Subparagraph 2 -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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