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Long-Term Contractual Arrangements (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year $ 684.5    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 590.6    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 442.3    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 376.2    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 344.9    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 2,371.7    
Unrecorded Unconditional Purchase Obligation, Total 4,810.2    
Supply And Stranded Costs Obligations Incurred 147.6 $ 99.2 $ 141.0
Natural Gas Procurement Costs Incurred 428.6 482.5 349.8
Fuel Costs Incurred 95.9 120.5 112.6
Transmission Support Commitment Costs Incurred 25.3 25.0 24.9
Peaker CfD Expense Incurred 42.7 18.1 51.9
Renewable Energy Costs Incurred 144.3 114.4 91.3
Supply And Stranded Cost Obligations [Member]      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 177.4    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 110.1    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 81.5    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 51.1    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 34.9    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 80.6    
Unrecorded Unconditional Purchase Obligation, Total 535.6    
Renewable Energy Contracts [Member]      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 246.6    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 273.3    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 238.3    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 237.4    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 237.0    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 2,174.7    
Unrecorded Unconditional Purchase Obligation, Total 3,407.3    
Peaker Contracts For Differences [Member]      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 55.8    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 41.1    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 20.4    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 7.8    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 4.0    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 3.6    
Unrecorded Unconditional Purchase Obligation, Total 132.7    
Natural Gas Procurement Contracts [Member]      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 137.9    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 123.8    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 78.4    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 57.8    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 46.9    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 99.7    
Unrecorded Unconditional Purchase Obligation, Total 544.5    
Wood, Coal And Transporation Contracts [Member]      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 45.4    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 23.3    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 3.4    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 1.9    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 1.9    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 13.1    
Unrecorded Unconditional Purchase Obligation, Total 89.0    
Transmission Support Commitments [Member]      
Long-term Purchase Commitment [Line Items]      
Recorded Unconditional Purchase Obligation Due within One Year 21.4    
Recorded Unconditional Purchase Obligation Due in Second Year 19.0    
Recorded Unconditional Purchase Obligation Due in Third Year 20.3    
Recorded Unconditional Purchase Obligation Due in Fourth Year 20.2    
Recorded Unconditional Purchase Obligation Due in Fifth Year 20.2    
Recorded Unconditional Purchase Obligation Due after Fifth Year 0.0    
Recorded Unconditional Purchase Obligation, Total 101.1    
The Connecticut Light And Power Company [Member]      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 279.4    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 207.9    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 159.5    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 126.9    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 114.5    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 711.6    
Unrecorded Unconditional Purchase Obligation, Total 1,599.8    
Supply And Stranded Costs Obligations Incurred 120.3 63.0 77.6
Transmission Support Commitment Costs Incurred 10.0 9.9 9.8
Peaker CfD Expense Incurred 42.7 18.1 51.9
Renewable Energy Costs Incurred 20.0 0.7  
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 145.0    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 87.2    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 58.2    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 38.0    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 29.3    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 47.6    
Unrecorded Unconditional Purchase Obligation, Total 405.3    
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 70.1    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 71.7    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 72.1    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 72.3    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 72.4    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 649.7    
Unrecorded Unconditional Purchase Obligation, Total 1,008.3    
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 55.8    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 41.1    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 20.4    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 7.8    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 4.0    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 3.6    
Unrecorded Unconditional Purchase Obligation, Total 132.7    
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 8.4    
Recorded Unconditional Purchase Obligation Due in Second Year 7.5    
Recorded Unconditional Purchase Obligation Due in Third Year 8.0    
Recorded Unconditional Purchase Obligation Due in Fourth Year 8.0    
Recorded Unconditional Purchase Obligation Due in Fifth Year 8.0    
Recorded Unconditional Purchase Obligation Due after Fifth Year 0.0    
Recorded Unconditional Purchase Obligation, Total 39.9    
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 0.1    
Recorded Unconditional Purchase Obligation Due in Second Year 0.4    
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 10.7    
Recorded Unconditional Purchase Obligation, Total 13.6    
NSTAR Electric Company [Member]      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 119.8    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 127.9    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 92.5    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 90.5    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 86.2    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 521.5    
Unrecorded Unconditional Purchase Obligation, Total 1,038.4    
Supply And Stranded Costs Obligations Incurred 6.5 7.0 32.4
Transmission Support Commitment Costs Incurred 7.8 7.7 7.7
Renewable Energy Costs Incurred 86.7 87.4 84.9
NSTAR Electric Company [Member] | Supply And Stranded Cost Obligations [Member]      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 14.1    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 4.8    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 5.5    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 5.5    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 3.1    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 28.1    
Unrecorded Unconditional Purchase Obligation, Total 61.1    
NSTAR Electric Company [Member] | Renewable Energy Contracts [Member]      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 99.0    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 117.0    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 80.4    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 78.5    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 76.6    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 489.8    
Unrecorded Unconditional Purchase Obligation, Total 941.3    
NSTAR Electric Company [Member] | Transmission Support Commitments [Member]      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 6.6    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 5.9    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 6.3    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 6.2    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 6.2    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 0.0    
Unrecorded Unconditional Purchase Obligation, Total 31.2    
NSTAR Electric Company [Member] | Yankee Companies Billings [Member]      
Long-term Purchase Commitment [Line Items]      
Recorded Unconditional Purchase Obligation Due within One Year 0.1    
Recorded Unconditional Purchase Obligation Due in Second Year 0.2    
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 3.6    
Recorded Unconditional Purchase Obligation, Total 4.8    
Public Service Company Of New Hampshire [Member]      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 136.3    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 114.6    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 95.9    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 84.8    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 81.0    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 882.6    
Unrecorded Unconditional Purchase Obligation, Total 1,395.2    
Supply And Stranded Costs Obligations Incurred 20.8 26.0 29.0
Fuel Costs Incurred 95.9 120.5 112.6
Transmission Support Commitment Costs Incurred 5.4 5.3 5.3
Renewable Energy Costs Incurred 37.2 26.3 6.4
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 18.3    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 18.1    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 17.8    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 7.6    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 2.5    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 4.9    
Unrecorded Unconditional Purchase Obligation, Total 69.2    
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 67.9    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 69.0    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 70.1    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 70.7    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 72.0    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 860.4    
Unrecorded Unconditional Purchase Obligation, Total 1,210.1    
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 45.4    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 23.3    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 3.4    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 1.9    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 1.9    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 13.1    
Unrecorded Unconditional Purchase Obligation, Total 89.0    
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 4.6    
Recorded Unconditional Purchase Obligation Due in Second Year 4.0    
Recorded Unconditional Purchase Obligation Due in Third Year 4.3    
Recorded Unconditional Purchase Obligation Due in Fourth Year 4.3    
Recorded Unconditional Purchase Obligation Due in Fifth Year 4.3    
Recorded Unconditional Purchase Obligation Due after Fifth Year 0.0    
Recorded Unconditional Purchase Obligation, Total 21.5    
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 0.1    
Recorded Unconditional Purchase Obligation Due in Second Year 0.2    
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 5.4    
Western Massachusetts Electric Company [Member]      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 11.4    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 17.3    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 17.6    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 17.8    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 17.9    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 177.5    
Unrecorded Unconditional Purchase Obligation, Total 259.5    
Supply And Stranded Costs Obligations Incurred   3.2 2.0
Transmission Support Commitment Costs Incurred 2.1 $ 2.1 $ 2.1
Renewable Energy Costs Incurred 0.4    
Western Massachusetts Electric Company [Member] | Renewable Energy Contracts [Member]      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 9.6    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 15.6    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 15.7    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 15.9    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 16.0    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 174.8    
Unrecorded Unconditional Purchase Obligation, Total 247.6    
Western Massachusetts Electric Company [Member] | Transmission Support Commitments [Member]      
Long-term Purchase Commitment [Line Items]      
Recorded Unconditional Purchase Obligation Due within One Year 1.8    
Recorded Unconditional Purchase Obligation Due in Second Year 1.6    
Recorded Unconditional Purchase Obligation Due in Third Year 1.7    
Recorded Unconditional Purchase Obligation Due in Fourth Year 1.7    
Recorded Unconditional Purchase Obligation Due in Fifth Year 1.7    
Recorded Unconditional Purchase Obligation Due after Fifth Year 0.0    
Recorded Unconditional Purchase Obligation, Total 8.5    
Western Massachusetts Electric Company [Member] | Yankee Companies Billings [Member]      
Long-term Purchase Commitment [Line Items]      
Recorded Unconditional Purchase Obligation Due within One Year 0.0    
Recorded Unconditional Purchase Obligation Due in Second Year 0.1    
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 2.7    
Recorded Unconditional Purchase Obligation, Total $ 3.4