Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Capital Expenditures Incurred But Not Yet Paid	$ 142.3	$ 125.5
The Connecticut Light And Power Company [Member]
Capital Expenditures Incurred But Not Yet Paid	47.2	54.0
NSTAR Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	29.5	21.6
Public Service Company Of New Hampshire [Member]
Capital Expenditures Incurred But Not Yet Paid	25.6	14.8
Western Massachusetts Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	$ 13.7	$ 9.9