Long-Term Contractual Arrangements (Details) (Renewable Energy Contracts [Member], USD $) In Millions, unless otherwise specified	Jun. 30, 2014
NSTAR Electric Company [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	$ 43.6
Unrecorded Unconditional Purchase Obligation, Due within Two Years	86.3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	93.7
Unrecorded Unconditional Purchase Obligation, Due within Four Years	89.8
Unrecorded Unconditional Purchase Obligation, Due within Five Years	53.3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	302.8
Unrecorded Unconditional Purchase Obligation, Total	669.5
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