Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Capital Expenditures Incurred But Not Yet Paid	$ 160.7	$ 128.9
The Connecticut Light And Power Company [Member]
Capital Expenditures Incurred But Not Yet Paid	46.0	52.2
NSTAR Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	31.2	18.1
Public Service Company Of New Hampshire [Member]
Capital Expenditures Incurred But Not Yet Paid	33.8	21.0
Western Massachusetts Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	$ 15.5	$ 10.0