Supplemental Cash Flow Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Capital Expenditures Incurred But Not Yet Paid	$ 110.4	$ 108.5
The Connecticut Light And Power Company [Member]
Capital Expenditures Incurred But Not Yet Paid	42.3	36.2
NSTAR Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	21.9	28.0
Public Service Company Of New Hampshire [Member]
Capital Expenditures Incurred But Not Yet Paid	21.7	14.4
Western Massachusetts Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	$ 8.3	$ 14.4