Supplemental Cash Flow Information (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Capital Expenditures Incurred But Not Yet Paid	$ 122.9	$ 139.9
The Connecticut Light And Power Company [Member]
Capital Expenditures Incurred But Not Yet Paid	36.6	45.9
NSTAR Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	31.9	21.5
Public Service Company Of New Hampshire [Member]
Capital Expenditures Incurred But Not Yet Paid	16.9	20.1
Western Massachusetts Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	$ 13.8	$ 35.1