LEASES - Future Minimum Lease Payments under Long-Term Leases (Details) $ in Millions	Mar. 31, 2019 USD ($)
Operating Leases
April 1, 2019 through December 31, 2019	$ 8.3
Year Ending December 31,
2020	9.6
2021	8.7
2022	7.3
2023	4.8
2024	2.8
Thereafter	29.7
Future lease payments	71.2
Less amount representing interest	14.5
Present value of future minimum lease payments	56.7
Finance Leases
April 1, 2019 through December 31, 2019	2.5
Year Ending December 31,
2020	3.4
2021	2.9
2022	1.5
2023	0.7
2024	0.7
Thereafter	13.2
Future lease payments	24.9
Less amount representing interest	13.9
Present value of future minimum lease payments	11.0
CL&P
Operating Leases
April 1, 2019 through December 31, 2019	0.2
Year Ending December 31,
2020	0.2
2021	0.0
2022	0.0
2023	0.0
2024	0.0
Thereafter	0.1
Future lease payments	0.5
Less amount representing interest	0.0
Present value of future minimum lease payments	0.5
Finance Leases
April 1, 2019 through December 31, 2019	1.5
Year Ending December 31,
2020	2.0
2021	1.5
2022	0.0
2023	0.0
2024	0.0
Thereafter	0.0
Future lease payments	5.0
Less amount representing interest	0.8
Present value of future minimum lease payments	4.2
NSTAR Electric
Operating Leases
April 1, 2019 through December 31, 2019	1.4
Year Ending December 31,
2020	1.6
2021	1.6
2022	1.6
2023	1.6
2024	1.7
Thereafter	29.4
Future lease payments	38.9
Less amount representing interest	13.8
Present value of future minimum lease payments	25.1
Finance Leases
April 1, 2019 through December 31, 2019	0.4
Year Ending December 31,
2020	0.5
2021	0.5
2022	0.6
2023	0.6
2024	0.6
Thereafter	12.8
Future lease payments	16.0
Less amount representing interest	11.5
Present value of future minimum lease payments	4.5
PSNH
Operating Leases
April 1, 2019 through December 31, 2019	0.1
Year Ending December 31,
2020	0.1
2021	0.1
2022	0.1
2023	0.1
2024	0.1
Thereafter	0.1
Future lease payments	0.7
Less amount representing interest	0.2
Present value of future minimum lease payments	0.5
Finance Leases
April 1, 2019 through December 31, 2019	0.1
Year Ending December 31,
2020	0.1
2021	0.1
2022	0.1
2023	0.1
2024	0.1
Thereafter	0.4
Future lease payments	1.0
Less amount representing interest	0.1
Present value of future minimum lease payments	$ 0.9