Supplemental Cash Flow Information (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
Capital Expenditures Incurred But Not Yet Paid	$ 128.9	$ 122.9
The Connecticut Light And Power Company [Member]
Capital Expenditures Incurred But Not Yet Paid	52.2	36.6
NSTAR Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	18.1	31.9
Public Service Company Of New Hampshire [Member]
Capital Expenditures Incurred But Not Yet Paid	21.0	16.9
Western Massachusetts Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	$ 10.0	$ 13.8