Long-Term Contractual Arrangements (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013
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	$ 1.2
Unrecorded Unconditional Purchase Obligation, Due within Two Years	49.9
Unrecorded Unconditional Purchase Obligation, Due within Three Years	50.9
Unrecorded Unconditional Purchase Obligation, Due within Four Years	51.4
Unrecorded Unconditional Purchase Obligation, Due within Five Years	52.0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	626.0
Unrecorded Unconditional Purchase Obligation, Total	831.4
The Connecticut Light And Power Company [Member] | Yankee Companies Billings [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	1.5
Unrecorded Unconditional Purchase Obligation, Due within Three Years	1.3
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0.8
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0.8
Unrecorded Unconditional Purchase Obligation, Due after Five Years	13.1
Unrecorded Unconditional Purchase Obligation, Total	17.9
NSTAR Electric Company [Member] | Yankee Companies Billings [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	0.2
Unrecorded Unconditional Purchase Obligation, Due within Two Years	0.7
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0.5
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0.2
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0.3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	4.5
Unrecorded Unconditional Purchase Obligation, Total	6.4
Public Service Company Of New Hampshire [Member] | Yankee Companies Billings [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	0.1
Unrecorded Unconditional Purchase Obligation, Due within Two Years	0.3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0.4
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0.3
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0.3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	5.2
Unrecorded Unconditional Purchase Obligation, Total	6.6
Western Massachusetts Electric Company [Member] | Yankee Companies Billings [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	0.1
Unrecorded Unconditional Purchase Obligation, Due within Two Years	0.4
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0.4
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0.2
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0.2
Unrecorded Unconditional Purchase Obligation, Due after Five Years	3.3
Unrecorded Unconditional Purchase Obligation, Total	$ 4.6