LEASES (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating Leased Assets [Line Items]
Capital Leases, Future Minimum Payments Due, Current	$ 2.8
Capital Leases, Future Minimum Payments Due in Two Years	2.2
Capital Leases, Future Minimum Payments Due in Three Years	2.2
Capital Leases, Future Minimum Payments Due in Four Years	2.0
Capital Leases, Future Minimum Payments Due in Five Years	2.0
Capital Leases, Future Minimum Payments Due Thereafter	7.5
Capital Leases, Future Minimum Payments Due	18.7
Capital Leases, Future Minimum Payments, Interest Included in Payments	7.6
Capital Leases, Future Minimum Payments, Present Value of Net Minimum Payments	11.1
Operating Leases, Rent Expense	14.8	8.4	11.9
Operating Leases, Future Minimum Payments Due, Current	22.4
Operating Leases, Future Minimum Payments, Due in Two Years	16.6
Operating Leases, Future Minimum Payments, Due in Three Years	14.1
Operating Leases, Future Minimum Payments, Due in Four Years	11.2
Operating Leases, Future Minimum Payments, Due in Five Years	8.6
Operating Leases, Future Minimum Payments, Due Thereafter	23.3
Operating Leases, Future Minimum Payments Due	96.2
The Connecticut Light And Power Company [Member]
Operating Leased Assets [Line Items]
Capital Leases, Future Minimum Payments Due, Current	2.2
Capital Leases, Future Minimum Payments Due in Two Years	2.0
Capital Leases, Future Minimum Payments Due in Three Years	2.0
Capital Leases, Future Minimum Payments Due in Four Years	1.9
Capital Leases, Future Minimum Payments Due in Five Years	2.0
Capital Leases, Future Minimum Payments Due Thereafter	7.3
Capital Leases, Future Minimum Payments Due	17.4
Capital Leases, Future Minimum Payments, Interest Included in Payments	7.4
Capital Leases, Future Minimum Payments, Present Value of Net Minimum Payments	10.0
Operating Leases, Rent Expense	8.2	8.3	10.0
Operating Leases, Future Minimum Payments Due, Current	4.3
Operating Leases, Future Minimum Payments, Due in Two Years	3.7
Operating Leases, Future Minimum Payments, Due in Three Years	3.1
Operating Leases, Future Minimum Payments, Due in Four Years	2.3
Operating Leases, Future Minimum Payments, Due in Five Years	1.2
Operating Leases, Future Minimum Payments, Due Thereafter	6.4
Operating Leases, Future Minimum Payments Due	21.0
Public Service Company Of New Hampshire [Member]
Operating Leased Assets [Line Items]
Capital Leases, Future Minimum Payments Due, Current	0.5
Capital Leases, Future Minimum Payments Due in Two Years	0.2
Capital Leases, Future Minimum Payments Due in Three Years	0.2
Capital Leases, Future Minimum Payments Due in Four Years	0.1
Capital Leases, Future Minimum Payments Due in Five Years	0
Capital Leases, Future Minimum Payments Due Thereafter	0
Capital Leases, Future Minimum Payments Due	1.0
Capital Leases, Future Minimum Payments, Interest Included in Payments	0.1
Capital Leases, Future Minimum Payments, Present Value of Net Minimum Payments	0.9
Operating Leases, Rent Expense	2.5	2.1	2.2
Operating Leases, Future Minimum Payments Due, Current	1.3
Operating Leases, Future Minimum Payments, Due in Two Years	1.0
Operating Leases, Future Minimum Payments, Due in Three Years	0.8
Operating Leases, Future Minimum Payments, Due in Four Years	0.6
Operating Leases, Future Minimum Payments, Due in Five Years	0.5
Operating Leases, Future Minimum Payments, Due Thereafter	1.8
Operating Leases, Future Minimum Payments Due	6.0
Western Massachusetts Electric Company [Member]
Operating Leased Assets [Line Items]
Operating Leases, Rent Expense	3.0	2.8	2.8
Operating Leases, Future Minimum Payments Due, Current	2.7
Operating Leases, Future Minimum Payments, Due in Two Years	1.1
Operating Leases, Future Minimum Payments, Due in Three Years	0.6
Operating Leases, Future Minimum Payments, Due in Four Years	0.3
Operating Leases, Future Minimum Payments, Due in Five Years	0.2
Operating Leases, Future Minimum Payments, Due Thereafter	1.2
Operating Leases, Future Minimum Payments Due	6.1
NSTAR Electric Company [Member]
Operating Leased Assets [Line Items]
Operating Leases, Rent Expense	6.2	19.8	19.2
Operating Leases, Future Minimum Payments Due, Current	10.9
Operating Leases, Future Minimum Payments, Due in Two Years	8.2
Operating Leases, Future Minimum Payments, Due in Three Years	7.4
Operating Leases, Future Minimum Payments, Due in Four Years	6.1
Operating Leases, Future Minimum Payments, Due in Five Years	5.1
Operating Leases, Future Minimum Payments, Due Thereafter	10.3
Operating Leases, Future Minimum Payments Due	$ 48.0