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Long-Term Contractual Arrangements (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year $ 705.4    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 615.6    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 538.1    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 428.7    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 368.1    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 2,385.6    
Unrecorded Unconditional Purchase Obligation, Total 5,041.5    
Supply And Stranded Costs Obligations Incurred 141.0 216.8 156.0
Natural Gas Procurement Costs Incurred 349.8 243.1 191.7
Fuel Costs Incurred 112.6 105.2 113.2
Transmission Support Commitment Costs Incurred 24.9 24.8 18.1
Peaker CfD Expense Incurred 51.9 59.3 40.2
Renewable Energy Costs Incurred 91.3 48.7 5.1
Supply And Stranded Cost Obligations [Member]
      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 224.2    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 205.2    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 177.7    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 119.9    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 110.4    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 309.3    
Unrecorded Unconditional Purchase Obligation, Total 1,146.7    
Renewable Energy Contracts [Member]
      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 194.5    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 203.8    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 214.1    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 211.9    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 177.3    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 1,885.7    
Unrecorded Unconditional Purchase Obligation, Total 2,887.3    
Peaker Contracts For Differences [Member]
      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 49.1    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 46.9    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 44.3    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 36.2    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 28.6    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 24.7    
Unrecorded Unconditional Purchase Obligation, Total 229.8    
Natural Gas Procurement Contracts [Member]
      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 134.4    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 117.6    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 76.5    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 37.7    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 24.2    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 103.9    
Unrecorded Unconditional Purchase Obligation, Total 494.3    
Wood, Coal And Transporation Contracts [Member]
      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 75.3    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 15.2    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 5.0    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 5.0    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 5.0    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 16.8    
Unrecorded Unconditional Purchase Obligation, Total 122.3    
Transmission Support Commitments [Member]
      
Long-term Purchase Commitment [Line Items]      
Recorded Unconditional Purchase Obligation Due within One Year 27.9    
Recorded Unconditional Purchase Obligation Due in Second Year 26.9    
Recorded Unconditional Purchase Obligation Due in Third Year 20.5    
Recorded Unconditional Purchase Obligation Due in Fourth Year 18.0    
Recorded Unconditional Purchase Obligation Due in Fifth Year 22.6    
Recorded Unconditional Purchase Obligation Due after Fifth Year 45.2    
Recorded Unconditional Purchase Obligation, Total 161.1    
The Connecticut Light And Power Company [Member]
      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 256.1    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 249.9    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 247.2    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 191.1    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 176.8    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 872.6    
Unrecorded Unconditional Purchase Obligation, Total 1,993.7    
Supply And Stranded Costs Obligations Incurred 77.6 158.2 114.9
Transmission Support Commitment Costs Incurred 9.8 9.6 10.3
Peaker CfD Expense Incurred 51.9 59.3 40.2
The Connecticut Light And Power Company [Member] | Supply And Stranded Cost Obligations [Member]
      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 145.6    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 141.1    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 143.7    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 96.2    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 87.1    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 257.6    
Unrecorded Unconditional Purchase Obligation, Total 871.3    
The Connecticut Light And Power Company [Member] | Renewable Energy Contracts [Member]
      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 48.9    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 49.9    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 50.3    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 50.8    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 51.4    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 560.2    
Unrecorded Unconditional Purchase Obligation, Total 811.5    
The Connecticut Light And Power Company [Member] | Peaker Contracts For Differences [Member]
      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 49.1    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 46.9    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 44.3    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 36.2    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 28.6    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 24.7    
Unrecorded Unconditional Purchase Obligation, Total 229.8    
The Connecticut Light And Power Company [Member] | Transmission Support Commitments [Member]
      
Long-term Purchase Commitment [Line Items]      
Recorded Unconditional Purchase Obligation Due within One Year 11.0    
Recorded Unconditional Purchase Obligation Due in Second Year 10.6    
Recorded Unconditional Purchase Obligation Due in Third Year 8.1    
Recorded Unconditional Purchase Obligation Due in Fourth Year 7.1    
Recorded Unconditional Purchase Obligation Due in Fifth Year 8.9    
Recorded Unconditional Purchase Obligation Due after Fifth Year 17.8    
Recorded Unconditional Purchase Obligation, Total 63.5    
The Connecticut Light And Power Company [Member] | Yankee Companies Billings [Member]
      
Long-term Purchase Commitment [Line Items]      
Recorded Unconditional Purchase Obligation Due within One Year 1.5    
Recorded Unconditional Purchase Obligation Due in Second Year 1.4    
Recorded Unconditional Purchase Obligation Due in Third Year 0.8    
Recorded Unconditional Purchase Obligation Due in Fourth Year 0.8    
Recorded Unconditional Purchase Obligation Due in Fifth Year 0.8    
Recorded Unconditional Purchase Obligation Due after Fifth Year 12.3    
Recorded Unconditional Purchase Obligation, Total 17.6    
NSTAR Electric Company [Member]
      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 132.7    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 131.3    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 108.3    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 93.4    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 58.2    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 262.5    
Unrecorded Unconditional Purchase Obligation, Total 786.4    
Supply And Stranded Costs Obligations Incurred 32.4 36.3 80.9
Transmission Support Commitment Costs Incurred 7.7 7.6 8.1
Renewable Energy Costs Incurred 84.9 60.2 61.8
NSTAR Electric Company [Member] | Supply And Stranded Cost Obligations [Member]
      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 36.2    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 36.1    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 15.8    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 5.6    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 5.5    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 36.8    
Unrecorded Unconditional Purchase Obligation, Total 136.0    
NSTAR Electric Company [Member] | Renewable Energy Contracts [Member]
      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 87.1    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 86.3    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 85.8    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 81.9    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 45.4    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 207.4    
Unrecorded Unconditional Purchase Obligation, Total 593.9    
NSTAR Electric Company [Member] | Transmission Support Commitments [Member]
      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 8.7    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 8.4    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 6.4    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 5.6    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 7.0    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 14.1    
Unrecorded Unconditional Purchase Obligation, Total 50.2    
NSTAR Electric Company [Member] | Yankee Companies Billings [Member]
      
Long-term Purchase Commitment [Line Items]      
Recorded Unconditional Purchase Obligation Due within One Year 0.7    
Recorded Unconditional Purchase Obligation Due in Second Year 0.5    
Recorded Unconditional Purchase Obligation Due in Third Year 0.3    
Recorded Unconditional Purchase Obligation Due in Fourth Year 0.3    
Recorded Unconditional Purchase Obligation Due in Fifth Year 0.3    
Recorded Unconditional Purchase Obligation Due after Fifth Year 4.2    
Recorded Unconditional Purchase Obligation, Total 6.3    
Public Service Company Of New Hampshire [Member]
      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 180.7    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 106.9    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 95.7    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 96.2    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 98.0    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 1,041.4    
Unrecorded Unconditional Purchase Obligation, Total 1,618.9    
Supply And Stranded Costs Obligations Incurred 29.0 30.5 40.8
Fuel Costs Incurred 112.6 105.2 113.2
Transmission Support Commitment Costs Incurred 5.3 5.2 5.6
Renewable Energy Costs Incurred 6.4 4.1 5.1
Public Service Company Of New Hampshire [Member] | Supply And Stranded Cost Obligations [Member]
      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 42.4    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 28.0    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 18.2    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 18.1    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 17.8    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 14.9    
Unrecorded Unconditional Purchase Obligation, Total 139.4    
Public Service Company Of New Hampshire [Member] | Renewable Energy Contracts [Member]
      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 56.8    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 57.7    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 67.9    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 69.0    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 70.1    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 995.2    
Unrecorded Unconditional Purchase Obligation, Total 1,316.7    
Public Service Company Of New Hampshire [Member] | Wood, Coal And Transporation Contracts [Member]
      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 75.3    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 15.2    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 5.0    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 5.0    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 5.0    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 16.8    
Unrecorded Unconditional Purchase Obligation, Total 122.3    
Public Service Company Of New Hampshire [Member] | Transmission Support Commitments [Member]
      
Long-term Purchase Commitment [Line Items]      
Recorded Unconditional Purchase Obligation Due within One Year 5.9    
Recorded Unconditional Purchase Obligation Due in Second Year 5.7    
Recorded Unconditional Purchase Obligation Due in Third Year 4.3    
Recorded Unconditional Purchase Obligation Due in Fourth Year 3.8    
Recorded Unconditional Purchase Obligation Due in Fifth Year 4.8    
Recorded Unconditional Purchase Obligation Due after Fifth Year 9.6    
Recorded Unconditional Purchase Obligation, Total 34.1    
Public Service Company Of New Hampshire [Member] | Yankee Companies Billings [Member]
      
Long-term Purchase Commitment [Line Items]      
Recorded Unconditional Purchase Obligation Due within One Year 0.3    
Recorded Unconditional Purchase Obligation Due in Second Year 0.3    
Recorded Unconditional Purchase Obligation Due in Third Year 0.3    
Recorded Unconditional Purchase Obligation Due in Fourth Year 0.3    
Recorded Unconditional Purchase Obligation Due in Fifth Year 0.3    
Recorded Unconditional Purchase Obligation Due after Fifth Year 4.9    
Recorded Unconditional Purchase Obligation, Total 6.4    
Western Massachusetts Electric Company [Member]
      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 4.4    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 12.5    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 12.0    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 11.9    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 12.5    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 129.7    
Unrecorded Unconditional Purchase Obligation, Total 183.0    
Supply And Stranded Costs Obligations Incurred 2.0 0.9 0.3
Transmission Support Commitment Costs Incurred 2.1 2.0 2.2
Western Massachusetts Electric Company [Member] | Renewable Energy Contracts [Member]
      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 1.7    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 9.9    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 10.1    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 10.2    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 10.4    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 122.9    
Unrecorded Unconditional Purchase Obligation, Total 165.2    
Western Massachusetts Electric Company [Member] | Transmission Support Commitments [Member]
      
Long-term Purchase Commitment [Line Items]      
Recorded Unconditional Purchase Obligation Due within One Year 2.3    
Recorded Unconditional Purchase Obligation Due in Second Year 2.2    
Recorded Unconditional Purchase Obligation Due in Third Year 1.7    
Recorded Unconditional Purchase Obligation Due in Fourth Year 1.5    
Recorded Unconditional Purchase Obligation Due in Fifth Year 1.9    
Recorded Unconditional Purchase Obligation Due after Fifth Year 3.7    
Recorded Unconditional Purchase Obligation, Total 13.3    
Western Massachusetts Electric Company [Member] | Yankee Companies Billings [Member]
      
Long-term Purchase Commitment [Line Items]      
Recorded Unconditional Purchase Obligation Due within One Year 0.4    
Recorded Unconditional Purchase Obligation Due in Second Year 0.4    
Recorded Unconditional Purchase Obligation Due in Third Year 0.2    
Recorded Unconditional Purchase Obligation Due in Fourth Year 0.2    
Recorded Unconditional Purchase Obligation Due in Fifth Year 0.2    
Recorded Unconditional Purchase Obligation Due after Fifth Year 3.1    
Recorded Unconditional Purchase Obligation, Total $ 4.5