Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Capital Expenditures Incurred But Not Yet Paid	$ 186.3	$ 142.3
The Connecticut Light And Power Company [Member]
Capital Expenditures Incurred But Not Yet Paid	59.5	47.2
NSTAR Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	38.5	29.5
Public Service Company Of New Hampshire [Member]
Capital Expenditures Incurred But Not Yet Paid	31.1	25.6
Western Massachusetts Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	$ 14.6	$ 13.7