Supplemental Cash Flow Information (Details) (USD $) In Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2011	Dec. 31, 2010
Capital Expenditures Incurred But Not Yet Paid	$ 109.4	$ 127.9
The Connecticut Light and Power Company [Member]
Capital Expenditures Incurred But Not Yet Paid	19.4	46.2
Public Service Company of New Hampshire [Member]
Capital Expenditures Incurred But Not Yet Paid	29.6	35.8
Western Massachusetts Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	$ 39.7	$ 21.2