Supplemental Cash Flow Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Capital Expenditures Incurred But Not Yet Paid	$ 98.7	$ 138.5
The Connecticut Light And Power Company [Member]
Capital Expenditures Incurred But Not Yet Paid	28.2	36.6
NSTAR Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	30.7	29.5
Public Service Company Of New Hampshire [Member]
Capital Expenditures Incurred But Not Yet Paid	12.9	32.5
Western Massachusetts Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	$ 15.8	$ 56.6