Supplemental Cash Flow Information (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Capital Expenditures Incurred But Not Yet Paid	$ 109.5	$ 166.3
The Connecticut Light And Power Company [Member]
Capital Expenditures Incurred But Not Yet Paid	28.3	45.1
NSTAR Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	33.4	22.7
Public Service Company Of New Hampshire [Member]
Capital Expenditures Incurred But Not Yet Paid	15.5	25.7
Western Massachusetts Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	$ 17.0	$ 56.5