Long-Term Contractual Arrangements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	$ 613.5
Unrecorded Unconditional Purchase Obligation, Due within Two Years	536.2
Unrecorded Unconditional Purchase Obligation, Due within Three Years	567.4
Unrecorded Unconditional Purchase Obligation, Due within Four Years	508.1
Unrecorded Unconditional Purchase Obligation, Due within Five Years	493.0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	4,129.1
Unrecorded Unconditional Purchase Obligation	6,847.3
Supply And Stranded Costs Obligations Incurred	132.2	196.2	205.3
Transmission Support Commitment Costs Incurred	18.1	18.9	18.6
Yankee Companies Billings Incurred	26.6	33.0	26.9
Peaker CfD Expense Incurred	40.2	10.0
Capacity CfD Expense Incurred	23.8
Supply And Stranded Cost Obligations [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	260.9
Unrecorded Unconditional Purchase Obligation, Due within Two Years	239.5
Unrecorded Unconditional Purchase Obligation, Due within Three Years	193.6
Unrecorded Unconditional Purchase Obligation, Due within Four Years	174.8
Unrecorded Unconditional Purchase Obligation, Due within Five Years	179.0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	649.4
Unrecorded Unconditional Purchase Obligation	1,697.2
Renewable Energy Contracts [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	60.0
Unrecorded Unconditional Purchase Obligation, Due within Three Years	175.6
Unrecorded Unconditional Purchase Obligation, Due within Four Years	177.9
Unrecorded Unconditional Purchase Obligation, Due within Five Years	189.1
Unrecorded Unconditional Purchase Obligation, Due after Five Years	2,955.8
Unrecorded Unconditional Purchase Obligation	3,569.8
Peaker Contracts For Differences [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	70.5
Unrecorded Unconditional Purchase Obligation, Due within Two Years	78.2
Unrecorded Unconditional Purchase Obligation, Due within Three Years	76.1
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	360.2
Unrecorded Unconditional Purchase Obligation	729.2
Natural Gas Procurement Contracts [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	68.1
Unrecorded Unconditional Purchase Obligation, Due within Two Years	55.6
Unrecorded Unconditional Purchase Obligation, Due within Three Years	52.0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	36.8
Unrecorded Unconditional Purchase Obligation, Due within Five Years	31.7
Unrecorded Unconditional Purchase Obligation, Due after Five Years	73.3
Unrecorded Unconditional Purchase Obligation	317.5
Wood, Coal And Transporation Contracts [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	135.1
Unrecorded Unconditional Purchase Obligation, Due within Two Years	33.6
Unrecorded Unconditional Purchase Obligation, Due within Three Years	21.0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	2.4
Unrecorded Unconditional Purchase Obligation, Due within Five Years	1.9
Unrecorded Unconditional Purchase Obligation, Due after Five Years	19.3
Unrecorded Unconditional Purchase Obligation	213.3
Portland Natural Gas Transmission System Pipeline Commitments [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	3.1
Unrecorded Unconditional Purchase Obligation, Due within Two Years	3.1
Unrecorded Unconditional Purchase Obligation, Due within Three Years	3.1
Unrecorded Unconditional Purchase Obligation, Due within Four Years	3.1
Unrecorded Unconditional Purchase Obligation, Due within Five Years	3.1
Unrecorded Unconditional Purchase Obligation, Due after Five Years	6.7
Unrecorded Unconditional Purchase Obligation	22.2
Transmission Support Commitments [Member]
Long-term Purchase Commitment [Line Items]
Recorded Unconditional Purchase Obligation Due within One Year	21.3
Recorded Unconditional Purchase Obligation Due in Second Year	20.2
Recorded Unconditional Purchase Obligation Due in Third Year	18.8
Recorded Unconditional Purchase Obligation Due in Fourth Year	18.6
Recorded Unconditional Purchase Obligation Due in Fifth Year	16.1
Recorded Unconditional Purchase Obligation Due after Fifth Year	64.4
Recorded Unconditional Purchase Obligation	159.4
Yankee Companies Billings [Member]
Long-term Purchase Commitment [Line Items]
Recorded Unconditional Purchase Obligation Due within One Year	27.3
Recorded Unconditional Purchase Obligation Due in Second Year	27.8
Recorded Unconditional Purchase Obligation Due in Third Year	27.2
Recorded Unconditional Purchase Obligation Due in Fourth Year	22.4
Recorded Unconditional Purchase Obligation Due in Fifth Year	0
Recorded Unconditional Purchase Obligation Due after Fifth Year	0
Recorded Unconditional Purchase Obligation	104.7
Select Energy Purchase Agreements [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	15.8
Unrecorded Unconditional Purchase Obligation, Due within Two Years	18.2
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0
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	34.0
The Connecticut Light And Power Company [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	282.6
Unrecorded Unconditional Purchase Obligation, Due within Two Years	324.0
Unrecorded Unconditional Purchase Obligation, Due within Three Years	362.2
Unrecorded Unconditional Purchase Obligation, Due within Four Years	352.1
Unrecorded Unconditional Purchase Obligation, Due within Five Years	349.5
Unrecorded Unconditional Purchase Obligation, Due after Five Years	2,577.1
Unrecorded Unconditional Purchase Obligation	4,247.5
Supply And Stranded Costs Obligations Incurred	91.1	151.3	173.1
Transmission Support Commitment Costs Incurred	10.3	10.8	10.7
Yankee Companies Billings Incurred	18.3	22.7	18.2
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	175.3
Unrecorded Unconditional Purchase Obligation, Due within Two Years	169.4
Unrecorded Unconditional Purchase Obligation, Due within Three Years	150.0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	145.6
Unrecorded Unconditional Purchase Obligation, Due within Five Years	159.6
Unrecorded Unconditional Purchase Obligation, Due after Five Years	595.3
Unrecorded Unconditional Purchase Obligation	1,395.2
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	5.9
Unrecorded Unconditional Purchase Obligation, Due within Two Years	45.8
Unrecorded Unconditional Purchase Obligation, Due within Three Years	106.6
Unrecorded Unconditional Purchase Obligation, Due within Four Years	107.9
Unrecorded Unconditional Purchase Obligation, Due within Five Years	108.6
Unrecorded Unconditional Purchase Obligation, Due after Five Years	1,584.7
Unrecorded Unconditional Purchase Obligation	1,959.5
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	70.5
Unrecorded Unconditional Purchase Obligation, Due within Two Years	78.2
Unrecorded Unconditional Purchase Obligation, Due within Three Years	76.1
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	360.2
Unrecorded Unconditional Purchase Obligation	729.2
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	12.2
Recorded Unconditional Purchase Obligation Due in Second Year	11.5
Recorded Unconditional Purchase Obligation Due in Third Year	10.8
Recorded Unconditional Purchase Obligation Due in Fourth Year	10.7
Recorded Unconditional Purchase Obligation Due in Fifth Year	9.2
Recorded Unconditional Purchase Obligation Due after Fifth Year	36.9
Recorded Unconditional Purchase Obligation	91.3
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	18.7
Recorded Unconditional Purchase Obligation Due in Second Year	19.1
Recorded Unconditional Purchase Obligation Due in Third Year	18.7
Recorded Unconditional Purchase Obligation Due in Fourth Year	15.8
Recorded Unconditional Purchase Obligation Due in Fifth Year	0
Recorded Unconditional Purchase Obligation Due after Fifth Year	0
Recorded Unconditional Purchase Obligation	72.3
Public Service Company Of New Hampshire [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	238.9
Unrecorded Unconditional Purchase Obligation, Due within Two Years	121.7
Unrecorded Unconditional Purchase Obligation, Due within Three Years	136.6
Unrecorded Unconditional Purchase Obligation, Due within Four Years	103.4
Unrecorded Unconditional Purchase Obligation, Due within Five Years	100.3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	1,363.2
Unrecorded Unconditional Purchase Obligation	2,064.1
Supply And Stranded Costs Obligations Incurred	40.8	42.6	29.8
Fuel Costs Incurred	110.5	168.3	156.7
Portland Natural Gas Transmission System Costs Incurred	2.7	2.8	1.6
Transmission Support Commitment Costs Incurred	5.6	5.8	5.7
Yankee Companies Billings Incurred	3.3	4.1	3.7
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	85.6
Unrecorded Unconditional Purchase Obligation, Due within Two Years	70.1
Unrecorded Unconditional Purchase Obligation, Due within Three Years	43.6
Unrecorded Unconditional Purchase Obligation, Due within Four Years	29.2
Unrecorded Unconditional Purchase Obligation, Due within Five Years	19.4
Unrecorded Unconditional Purchase Obligation, Due after Five Years	54.1
Unrecorded Unconditional Purchase Obligation	302.0
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	5.1
Unrecorded Unconditional Purchase Obligation, Due within Two Years	5.1
Unrecorded Unconditional Purchase Obligation, Due within Three Years	59.8
Unrecorded Unconditional Purchase Obligation, Due within Four Years	60.7
Unrecorded Unconditional Purchase Obligation, Due within Five Years	70.9
Unrecorded Unconditional Purchase Obligation, Due after Five Years	1,263.3
Unrecorded Unconditional Purchase Obligation	1,464.9
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	135.1
Unrecorded Unconditional Purchase Obligation, Due within Two Years	33.6
Unrecorded Unconditional Purchase Obligation, Due within Three Years	21.0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	2.4
Unrecorded Unconditional Purchase Obligation, Due within Five Years	1.9
Unrecorded Unconditional Purchase Obligation, Due after Five Years	19.3
Unrecorded Unconditional Purchase Obligation	213.3
Public Service Company Of New Hampshire [Member] | Portland Natural Gas Transmission System Pipeline Commitments [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	3.1
Unrecorded Unconditional Purchase Obligation, Due within Two Years	3.1
Unrecorded Unconditional Purchase Obligation, Due within Three Years	3.1
Unrecorded Unconditional Purchase Obligation, Due within Four Years	3.1
Unrecorded Unconditional Purchase Obligation, Due within Five Years	3.1
Unrecorded Unconditional Purchase Obligation, Due after Five Years	6.7
Unrecorded Unconditional Purchase Obligation	22.2
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	6.6
Recorded Unconditional Purchase Obligation Due in Second Year	6.3
Recorded Unconditional Purchase Obligation Due in Third Year	5.8
Recorded Unconditional Purchase Obligation Due in Fourth Year	5.7
Recorded Unconditional Purchase Obligation Due in Fifth Year	5.0
Recorded Unconditional Purchase Obligation Due after Fifth Year	19.8
Recorded Unconditional Purchase Obligation	49.2
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	3.4
Recorded Unconditional Purchase Obligation Due in Second Year	3.5
Recorded Unconditional Purchase Obligation Due in Third Year	3.3
Recorded Unconditional Purchase Obligation Due in Fourth Year	2.3
Recorded Unconditional Purchase Obligation Due in Fifth Year	0
Recorded Unconditional Purchase Obligation Due after Fifth Year	0
Recorded Unconditional Purchase Obligation	12.5
Western Massachusetts Electric Company [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	16.7
Unrecorded Unconditional Purchase Obligation, Due within Three Years	16.6
Unrecorded Unconditional Purchase Obligation, Due within Four Years	15.8
Unrecorded Unconditional Purchase Obligation, Due within Five Years	11.5
Unrecorded Unconditional Purchase Obligation, Due after Five Years	115.5
Unrecorded Unconditional Purchase Obligation	184.2
Supply And Stranded Costs Obligations Incurred	0.3	2.3	2.4
Transmission Support Commitment Costs Incurred	2.2	2.3	2.2
Yankee Companies Billings Incurred	5.0	6.2	5.0
Western Massachusetts Electric Company [Member] | Renewable Energy Contracts [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	0.4
Unrecorded Unconditional Purchase Obligation, Due within Two Years	9.1
Unrecorded Unconditional Purchase Obligation, Due within Three Years	9.2
Unrecorded Unconditional Purchase Obligation, Due within Four Years	9.3
Unrecorded Unconditional Purchase Obligation, Due within Five Years	9.6
Unrecorded Unconditional Purchase Obligation, Due after Five Years	107.8
Unrecorded Unconditional Purchase Obligation	145.4
Western Massachusetts Electric Company [Member] | Transmission Support Commitments [Member]
Long-term Purchase Commitment [Line Items]
Recorded Unconditional Purchase Obligation Due within One Year	2.5
Recorded Unconditional Purchase Obligation Due in Second Year	2.4
Recorded Unconditional Purchase Obligation Due in Third Year	2.2
Recorded Unconditional Purchase Obligation Due in Fourth Year	2.2
Recorded Unconditional Purchase Obligation Due in Fifth Year	1.9
Recorded Unconditional Purchase Obligation Due after Fifth Year	7.7
Recorded Unconditional Purchase Obligation	18.9
Western Massachusetts Electric Company [Member] | Yankee Companies Billings [Member]
Long-term Purchase Commitment [Line Items]
Recorded Unconditional Purchase Obligation Due within One Year	5.2
Recorded Unconditional Purchase Obligation Due in Second Year	5.2
Recorded Unconditional Purchase Obligation Due in Third Year	5.2
Recorded Unconditional Purchase Obligation Due in Fourth Year	4.3
Recorded Unconditional Purchase Obligation Due in Fifth Year	0
Recorded Unconditional Purchase Obligation Due after Fifth Year	0
Recorded Unconditional Purchase Obligation	19.9
Yankee Gas Services Company [Member]
Long-term Purchase Commitment [Line Items]
Natural Gas Procurement Costs Incurred	$ 191.7	$ 209.5	$ 236.3