LEASES (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Operating Leased Assets [Line Items]
Capital Leases, Future Minimum Payments Due in Two Years	$ 2.2
Capital Leases, Future Minimum Payments Due in Three Years	2.1
Capital Leases, Future Minimum Payments Due in Four Years	2.1
Capital Leases, Future Minimum Payments Due in Five Years	2.0
Capital Leases, Future Minimum Payments Due Thereafter	1.4
Capital Leases, Future Minimum Payments Due	11.8
Capital Leases, Future Minimum Payments, Interest Included in Payments	3.6
Capital Leases, Future Minimum Payments, Present Value of Net Minimum Payments	8.2
Operating Leases, Rent Expense	16.3
Operating Leases, Future Minimum Payments, Due in Two Years	16.4
Operating Leases, Future Minimum Payments, Due in Three Years	13.8
Operating Leases, Future Minimum Payments, Due in Four Years	10.4
Operating Leases, Future Minimum Payments, Due in Five Years	8.5
Operating Leases, Future Minimum Payments, Due Thereafter	15.4
Operating Leases, Future Minimum Payments Due	71.3
The Connecticut Light And Power Company [Member]
Operating Leased Assets [Line Items]
Capital Leases, Future Minimum Payments Due in Two Years	1.9
Capital Leases, Future Minimum Payments Due in Three Years	1.9
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	1.4
Capital Leases, Future Minimum Payments Due	11.2
Capital Leases, Future Minimum Payments, Interest Included in Payments	3.6
Capital Leases, Future Minimum Payments, Present Value of Net Minimum Payments	7.6
Operating Leases, Rent Expense	8.1
Operating Leases, Future Minimum Payments, Due in Two Years	2.9
Operating Leases, Future Minimum Payments, Due in Three Years	2.0
Operating Leases, Future Minimum Payments, Due in Four Years	1.3
Operating Leases, Future Minimum Payments, Due in Five Years	1.0
Operating Leases, Future Minimum Payments, Due Thereafter	1.7
Operating Leases, Future Minimum Payments Due	9.6
NSTAR Electric Company [Member]
Operating Leased Assets [Line Items]
Operating Leases, Rent Expense	7.8	$ 6.7
Operating Leases, Future Minimum Payments, Due in Two Years	9.7
Operating Leases, Future Minimum Payments, Due in Three Years	8.5
Operating Leases, Future Minimum Payments, Due in Four Years	6.5
Operating Leases, Future Minimum Payments, Due in Five Years	5.3
Operating Leases, Future Minimum Payments, Due Thereafter	9.0
Operating Leases, Future Minimum Payments Due	43.3
Public Service Company Of New Hampshire [Member]
Operating Leased Assets [Line Items]
Capital Leases, Future Minimum Payments Due in Two Years	0.3
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	0.0
Capital Leases, Future Minimum Payments Due Thereafter	0.0
Capital Leases, Future Minimum Payments Due	0.6
Capital Leases, Future Minimum Payments, Interest Included in Payments	0.0
Capital Leases, Future Minimum Payments, Present Value of Net Minimum Payments	0.6
Operating Leases, Rent Expense	1.7
Operating Leases, Future Minimum Payments, Due in Two Years	0.8
Operating Leases, Future Minimum Payments, Due in Three Years	0.7
Operating Leases, Future Minimum Payments, Due in Four Years	0.5
Operating Leases, Future Minimum Payments, Due in Five Years	0.4
Operating Leases, Future Minimum Payments, Due Thereafter	0.7
Operating Leases, Future Minimum Payments Due	3.4
Western Massachusetts Electric Company [Member]
Operating Leased Assets [Line Items]
Operating Leases, Rent Expense	1.2	$ 2.9
Operating Leases, Future Minimum Payments, Due in Two Years	0.8
Operating Leases, Future Minimum Payments, Due in Three Years	0.7
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	$ 4.9