Supplemental Cash Flow Information (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Capital Expenditures Incurred But Not Yet Paid	$ 166.3	$ 109.4
The Connecticut Light And Power Company [Member]
Capital Expenditures Incurred But Not Yet Paid	45.1	19.4
Storm Reserve Credits Paid To Customers	27.0
Public Service Company Of New Hampshire [Member]
Capital Expenditures Incurred But Not Yet Paid	25.7	29.6
Western Massachusetts Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	56.5	39.7
NSTAR Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	$ 22.7	$ 18.8