Long-Term Contractual Arrangements (Details) (USD $) In Millions, unless otherwise specified	Jun. 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	$ 4.6
Unrecorded Unconditional Purchase Obligation, Due within Two Years	49.4
Unrecorded Unconditional Purchase Obligation, Due within Three Years	49.9
Unrecorded Unconditional Purchase Obligation, Due within Four Years	50.4
Unrecorded Unconditional Purchase Obligation, Due within Five Years	51.0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	607.0
Unrecorded Unconditional Purchase Obligation, Total	812.3
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	1.0
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	18.5
NSTAR Electric Company [Member] | Yankee Companies Billings [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	0.5
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	6.7
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	0.3
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	6.8
Western Massachusetts Electric Company [Member] | Yankee Companies Billings [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	0.3
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	$ 4.8