Long-Term Contractual Arrangements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	$ 685.8
Unrecorded Unconditional Purchase Obligation, Due within Two Years	598.2
Unrecorded Unconditional Purchase Obligation, Due within Three Years	421.6
Unrecorded Unconditional Purchase Obligation, Due within Four Years	327.6
Unrecorded Unconditional Purchase Obligation, Due within Five Years	290.0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	2,198.4
Unrecorded Unconditional Purchase Obligation, Total	4,521.6
Supply And Stranded Costs Obligations Incurred	99.2	141.0	216.8
Natural Gas Procurement Costs Incurred	482.5	349.8	243.1
Fuel Costs Incurred	120.5	112.6	105.2
Transmission Support Commitment Costs Incurred	25.0	24.9	24.8
Peaker CfD Expense Incurred	18.1	51.9	59.3
Renewable Energy Costs Incurred	114.4	91.3	48.7
Supply And Stranded Cost Obligations [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	196.6
Unrecorded Unconditional Purchase Obligation, Due within Two Years	169.2
Unrecorded Unconditional Purchase Obligation, Due within Three Years	101.9
Unrecorded Unconditional Purchase Obligation, Due within Four Years	65.3
Unrecorded Unconditional Purchase Obligation, Due within Five Years	38.1
Unrecorded Unconditional Purchase Obligation, Due after Five Years	82.4
Unrecorded Unconditional Purchase Obligation, Total	653.5
Renewable Energy Contracts [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	204.3
Unrecorded Unconditional Purchase Obligation, Due within Two Years	240.5
Unrecorded Unconditional Purchase Obligation, Due within Three Years	239.6
Unrecorded Unconditional Purchase Obligation, Due within Four Years	204.2
Unrecorded Unconditional Purchase Obligation, Due within Five Years	202.9
Unrecorded Unconditional Purchase Obligation, Due after Five Years	1,994.6
Unrecorded Unconditional Purchase Obligation, Total	3,086.1
Peaker Contracts For Differences [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	26.3
Unrecorded Unconditional Purchase Obligation, Due within Two Years	25.4
Unrecorded Unconditional Purchase Obligation, Due within Three Years	10.5
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation, Total	62.2
Natural Gas Procurement Contracts [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	133.7
Unrecorded Unconditional Purchase Obligation, Due within Two Years	116.3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	45.6
Unrecorded Unconditional Purchase Obligation, Due within Four Years	31.9
Unrecorded Unconditional Purchase Obligation, Due within Five Years	25.8
Unrecorded Unconditional Purchase Obligation, Due after Five Years	85.1
Unrecorded Unconditional Purchase Obligation, Total	438.4
Wood, Coal And Transporation 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	25.2
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	1.9
Unrecorded Unconditional Purchase Obligation, Due after Five Years	15.0
Unrecorded Unconditional Purchase Obligation, Total	151.1
Transmission Support Commitments [Member]
Long-term Purchase Commitment [Line Items]
Recorded Unconditional Purchase Obligation Due within One Year	25.9
Recorded Unconditional Purchase Obligation Due in Second Year	21.6
Recorded Unconditional Purchase Obligation Due in Third Year	19.0
Recorded Unconditional Purchase Obligation Due in Fourth Year	21.2
Recorded Unconditional Purchase Obligation Due in Fifth Year	21.3
Recorded Unconditional Purchase Obligation Due after Fifth Year	21.3
Recorded Unconditional Purchase Obligation, Total	130.3
The Connecticut Light And Power Company [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	233.4
Unrecorded Unconditional Purchase Obligation, Due within Two Years	241.8
Unrecorded Unconditional Purchase Obligation, Due within Three Years	169.1
Unrecorded Unconditional Purchase Obligation, Due within Four Years	123.4
Unrecorded Unconditional Purchase Obligation, Due within Five Years	106.4
Unrecorded Unconditional Purchase Obligation, Due after Five Years	779.7
Unrecorded Unconditional Purchase Obligation, Total	1,653.8
Supply And Stranded Costs Obligations Incurred	63.0	77.6	158.2
Transmission Support Commitment Costs Incurred	9.9	9.8	9.6
Peaker CfD Expense Incurred	18.1	51.9	59.3
Renewable Energy Costs Incurred	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	134.3
Unrecorded Unconditional Purchase Obligation, Due within Two Years	136.8
Unrecorded Unconditional Purchase Obligation, Due within Three Years	79.0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	42.0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	25.0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	43.6
Unrecorded Unconditional Purchase Obligation, Total	460.7
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	61.2
Unrecorded Unconditional Purchase Obligation, Due within Two Years	70.3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	71.3
Unrecorded Unconditional Purchase Obligation, Due within Four Years	72.1
Unrecorded Unconditional Purchase Obligation, Due within Five Years	72.1
Unrecorded Unconditional Purchase Obligation, Due after Five Years	715.9
Unrecorded Unconditional Purchase Obligation, Total	1,062.9
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	26.3
Unrecorded Unconditional Purchase Obligation, Due within Two Years	25.4
Unrecorded Unconditional Purchase Obligation, Due within Three Years	10.5
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation, Total	62.2
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	10.2
Recorded Unconditional Purchase Obligation Due in Second Year	8.5
Recorded Unconditional Purchase Obligation Due in Third Year	7.5
Recorded Unconditional Purchase Obligation Due in Fourth Year	8.4
Recorded Unconditional Purchase Obligation Due in Fifth Year	8.4
Recorded Unconditional Purchase Obligation Due after Fifth Year	8.4
Recorded Unconditional Purchase Obligation, Total	51.4
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	1.4
Recorded Unconditional Purchase Obligation Due in Second Year	0.8
Recorded Unconditional Purchase Obligation Due in Third Year	0.8
Recorded Unconditional Purchase Obligation Due in Fourth Year	0.9
Recorded Unconditional Purchase Obligation Due in Fifth Year	0.9
Recorded Unconditional Purchase Obligation Due after Fifth Year	11.8
Recorded Unconditional Purchase Obligation, Total	16.6
NSTAR Electric Company [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	128.3
Unrecorded Unconditional Purchase Obligation, Due within Two Years	121.0
Unrecorded Unconditional Purchase Obligation, Due within Three Years	107.9
Unrecorded Unconditional Purchase Obligation, Due within Four Years	72.0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	70.1
Unrecorded Unconditional Purchase Obligation, Due after Five Years	361.8
Unrecorded Unconditional Purchase Obligation, Total	861.1
Supply And Stranded Costs Obligations Incurred	7.0	32.4	36.3
Transmission Support Commitment Costs Incurred	7.7	7.7	7.6
Renewable Energy Costs Incurred	87.4	84.9	60.2
NSTAR Electric Company [Member] | Supply And Stranded Cost Obligations [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	34.3
Unrecorded Unconditional Purchase Obligation, Due within Two Years	14.1
Unrecorded Unconditional Purchase Obligation, Due within Three Years	4.8
Unrecorded Unconditional Purchase Obligation, Due within Four Years	5.5
Unrecorded Unconditional Purchase Obligation, Due within Five Years	5.5
Unrecorded Unconditional Purchase Obligation, Due after Five Years	31.4
Unrecorded Unconditional Purchase Obligation, Total	95.6
NSTAR Electric Company [Member] | Renewable Energy Contracts [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	85.4
Unrecorded Unconditional Purchase Obligation, Due within Two Years	99.9
Unrecorded Unconditional Purchase Obligation, Due within Three Years	96.9
Unrecorded Unconditional Purchase Obligation, Due within Four Years	59.6
Unrecorded Unconditional Purchase Obligation, Due within Five Years	57.7
Unrecorded Unconditional Purchase Obligation, Due after Five Years	319.8
Unrecorded Unconditional Purchase Obligation, Total	719.3
NSTAR Electric Company [Member] | Transmission Support Commitments [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	8.1
Unrecorded Unconditional Purchase Obligation, Due within Two Years	6.7
Unrecorded Unconditional Purchase Obligation, Due within Three Years	5.9
Unrecorded Unconditional Purchase Obligation, Due within Four Years	6.6
Unrecorded Unconditional Purchase Obligation, Due within Five Years	6.6
Unrecorded Unconditional Purchase Obligation, Due after Five Years	6.6
Unrecorded Unconditional Purchase Obligation, Total	40.5
NSTAR Electric Company [Member] | Yankee Companies Billings [Member]
Long-term Purchase Commitment [Line Items]
Recorded Unconditional Purchase Obligation Due within One Year	0.5
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	4.0
Recorded Unconditional Purchase Obligation, Total	5.7
Public Service Company Of New Hampshire [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	190.6
Unrecorded Unconditional Purchase Obligation, Due within Two Years	116.3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	96.4
Unrecorded Unconditional Purchase Obligation, Due within Four Years	97.7
Unrecorded Unconditional Purchase Obligation, Due within Five Years	85.0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	964.0
Unrecorded Unconditional Purchase Obligation, Total	1,550.0
Supply And Stranded Costs Obligations Incurred	26.0	29.0	30.5
Fuel Costs Incurred	120.5	112.6	105.2
Transmission Support Commitment Costs Incurred	5.3	5.3	5.2
Renewable Energy Costs Incurred	26.3	6.4	4.1
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	28.0
Unrecorded Unconditional Purchase Obligation, Due within Two Years	18.3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	18.1
Unrecorded Unconditional Purchase Obligation, Due within Four Years	17.8
Unrecorded Unconditional Purchase Obligation, Due within Five Years	7.6
Unrecorded Unconditional Purchase Obligation, Due after Five Years	7.4
Unrecorded Unconditional Purchase Obligation, Total	97.2
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	57.7
Unrecorded Unconditional Purchase Obligation, Due within Two Years	67.9
Unrecorded Unconditional Purchase Obligation, Due within Three Years	69.0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	70.1
Unrecorded Unconditional Purchase Obligation, Due within Five Years	70.7
Unrecorded Unconditional Purchase Obligation, Due after Five Years	932.4
Unrecorded Unconditional Purchase Obligation, Total	1,267.8
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	99.0
Unrecorded Unconditional Purchase Obligation, Due within Two Years	25.2
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	1.9
Unrecorded Unconditional Purchase Obligation, Due after Five Years	15.0
Unrecorded Unconditional Purchase Obligation, Total	151.1
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	5.5
Recorded Unconditional Purchase Obligation Due in Second Year	4.6
Recorded Unconditional Purchase Obligation Due in Third Year	4.0
Recorded Unconditional Purchase Obligation Due in Fourth Year	4.5
Recorded Unconditional Purchase Obligation Due in Fifth Year	4.5
Recorded Unconditional Purchase Obligation Due after Fifth Year	4.5
Recorded Unconditional Purchase Obligation, Total	27.6
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	0.4
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	4.7
Recorded Unconditional Purchase Obligation, Total	6.3
Western Massachusetts Electric Company [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	2.4
Unrecorded Unconditional Purchase Obligation, Due within Two Years	4.4
Unrecorded Unconditional Purchase Obligation, Due within Three Years	4.2
Unrecorded Unconditional Purchase Obligation, Due within Four Years	4.3
Unrecorded Unconditional Purchase Obligation, Due within Five Years	4.4
Unrecorded Unconditional Purchase Obligation, Due after Five Years	31.3
Unrecorded Unconditional Purchase Obligation, Total	51.0
Supply And Stranded Costs Obligations Incurred	3.2	2.0	0.9
Transmission Support Commitment Costs Incurred	2.1	2.1	2.0
Western Massachusetts Electric Company [Member] | Renewable Energy Contracts [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	0
Unrecorded Unconditional Purchase Obligation, Due within Two Years	2.4
Unrecorded Unconditional Purchase Obligation, Due within Three Years	2.4
Unrecorded Unconditional Purchase Obligation, Due within Four Years	2.4
Unrecorded Unconditional Purchase Obligation, Due within Five Years	2.4
Unrecorded Unconditional Purchase Obligation, Due after Five Years	26.5
Unrecorded Unconditional Purchase Obligation, Total	36.1
Western Massachusetts Electric Company [Member] | Transmission Support Commitments [Member]
Long-term Purchase Commitment [Line Items]
Recorded Unconditional Purchase Obligation Due within One Year	2.1
Recorded Unconditional Purchase Obligation Due in Second Year	1.8
Recorded Unconditional Purchase Obligation Due in Third Year	1.6
Recorded Unconditional Purchase Obligation Due in Fourth Year	1.7
Recorded Unconditional Purchase Obligation Due in Fifth Year	1.8
Recorded Unconditional Purchase Obligation Due after Fifth Year	1.8
Recorded Unconditional Purchase Obligation, Total	10.8
Western Massachusetts Electric Company [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	0.2
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	3.0
Recorded Unconditional Purchase Obligation, Total	$ 4.1