Supplemental Cash Flow Information (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Capital Expenditures Incurred But Not Yet Paid	$ 125.5	$ 109.5
Description of DOE Damages Claims Proceeds	In the first half of 2014, as a result of awards issued to the Yankee Companies for spent nuclear fuel lawsuits against the DOE described in Note 8C, "Commitments and Contingencies - Contractual Obligations - Yankee Companies," NU recognized total proceeds of $125.7 million, which were net of $80.6 million in proceeds CY and YAEC returned to non-affiliated member companies.
The Connecticut Light And Power Company [Member]
Capital Expenditures Incurred But Not Yet Paid	54.0	28.3
NSTAR Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	21.6	33.4
Public Service Company Of New Hampshire [Member]
Capital Expenditures Incurred But Not Yet Paid	14.8	15.5
Western Massachusetts Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	$ 9.9	$ 17.0