Supplemental Cash Flow Information (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Capital Expenditures Incurred But Not Yet Paid	$ 139.9	$ 121.7
The Connecticut Light And Power Company [Member]
Capital Expenditures Incurred But Not Yet Paid	45.9	22.1
Storm Reserve Credits Paid To Customers	27.0
Public Service Company Of New Hampshire [Member]
Capital Expenditures Incurred But Not Yet Paid	20.1	32.4
Western Massachusetts Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	35.1	45.8
NSTAR Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	$ 21.5	$ 29.4