LEASES - Future Minimum Rental Payments, Operating and Capital Leases (Details) $ in Millions	Dec. 31, 2017 USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018	$ 13.2
2019	11.4
2020	10.0
2021	8.9
2022	7.4
Thereafter	19.7
Future minimum lease payments	70.6
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018	2.9
2019	3.3
2020	3.3
2021	2.8
2022	1.3
Thereafter	2.5
Future minimum lease payments	16.1
Less amount representing interest	3.1
Present value of future minimum lease payments	13.0
The Connecticut Light And Power Company
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018	1.8
2019	1.5
2020	1.3
2021	1.1
2022	1.0
Thereafter	1.0
Future minimum lease payments	7.7
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018	2.0
2019	2.0
2020	2.0
2021	1.4
2022	0.0
Thereafter	0.0
Future minimum lease payments	7.4
Less amount representing interest	1.7
Present value of future minimum lease payments	5.7
NSTAR Electric Company
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018	7.9
2019	6.9
2020	6.1
2021	5.5
2022	4.5
Thereafter	15.4
Future minimum lease payments	46.3
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018	0.5
2019	0.6
2020	0.5
2021	0.6
2022	0.6
Thereafter	2.5
Future minimum lease payments	5.3
Less amount representing interest	1.2
Present value of future minimum lease payments	4.1
Public Service Company Of New Hampshire
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018	1.0
2019	1.0
2020	0.9
2021	0.8
2022	0.6
Thereafter	2.0
Future minimum lease payments	6.3
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018	0.1
2019	0.0
2020	0.0
2021	0.0
2022	0.0
Thereafter	0.0
Future minimum lease payments	0.1
Less amount representing interest	0.0
Present value of future minimum lease payments	$ 0.1