Supplemental Cash Flow Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Capital Expenditures Incurred But Not Yet Paid	$ 138.5	$ 168.5
The Connecticut Light And Power Company [Member]
Capital Expenditures Incurred But Not Yet Paid	36.6	32.7
Storm Reserve Credits Paid To Customers	27.0
Public Service Company Of New Hampshire [Member]
Capital Expenditures Incurred But Not Yet Paid	32.5	51.1
Western Massachusetts Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	$ 56.6	$ 61.3