Long-Term Contractual Arrangements (Details) (Renewable Energy Contracts [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2014
The Connecticut Light And Power Company [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	$ 20.0
Unrecorded Unconditional Purchase Obligation, Due within Two Years	60.7
Unrecorded Unconditional Purchase Obligation, Due within Three Years	66.1
Unrecorded Unconditional Purchase Obligation, Due within Four Years	67.0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	67.7
Unrecorded Unconditional Purchase Obligation, Due after Five Years	717.0
Unrecorded Unconditional Purchase Obligation, Total	998.5
NSTAR Electric Company [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	21.8
Unrecorded Unconditional Purchase Obligation, Due within Two Years	86.3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	100.0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	96.1
Unrecorded Unconditional Purchase Obligation, Due within Five Years	59.6
Unrecorded Unconditional Purchase Obligation, Due after Five Years	377.6
Unrecorded Unconditional Purchase Obligation, Total	741.4
Western Massachusetts Electric Company [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	0
Unrecorded Unconditional Purchase Obligation, Due within Two Years	0
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	28.9
Unrecorded Unconditional Purchase Obligation, Total	$ 36.1