LEASES - Future Minimum Rental Payments, Operating and Capital Leases (Details) $ in Millions	Dec. 31, 2018 USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019	$ 11.5
2020	9.8
2021	8.7
2022	7.2
2023	4.7
Thereafter	32.7
Future minimum lease payments	74.6
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019	3.4
2020	3.4
2021	2.9
2022	1.5
2023	0.7
Thereafter	13.9
Future minimum lease payments	25.8
Less amount to arrive at present value	13.8
Present value of future minimum lease payments	12.0
The Connecticut Light And Power Company
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019	1.5
2020	1.4
2021	1.2
2022	1.1
2023	0.5
Thereafter	0.2
Future minimum lease payments	5.9
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019	2.0
2020	2.0
2021	1.5
2022	0.0
2023	0.0
Thereafter	0.0
Future minimum lease payments	5.5
Less amount to arrive at present value	1.0
Present value of future minimum lease payments	4.5
NSTAR Electric Company
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019	7.2
2020	6.0
2021	5.3
2022	4.4
2023	3.1
Thereafter	29.5
Future minimum lease payments	55.5
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019	0.5
2020	0.5
2021	0.5
2022	0.6
2023	0.6
Thereafter	13.4
Future minimum lease payments	16.1
Less amount to arrive at present value	12.4
Present value of future minimum lease payments	3.7
Public Service Company Of New Hampshire
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019	0.5
2020	0.4
2021	0.4
2022	0.4
2023	0.2
Thereafter	0.3
Future minimum lease payments	2.2
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019	0.1
2020	0.1
2021	0.1
2022	0.1
2023	0.1
Thereafter	0.5
Future minimum lease payments	1.0
Less amount to arrive at present value	0.1
Present value of future minimum lease payments	$ 0.9