LEASES - Future Minimum Lease Payments under Long-Term Leases (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Operating Leases
2021	$ 11.4
2022	9.0
2023	6.4
2024	4.5
2025	3.4
Thereafter	36.2
Future lease payments	70.9
Less amount representing interest	15.7
Present value of future minimum lease payments	55.2	$ 49.9
Finance Leases
2021	7.2
2022	5.7
2023	4.9
2024	4.8
2025	4.7
Thereafter	60.7
Future lease payments	88.0
Less amount representing interest	25.4
Present value of future minimum lease payments	62.6	10.5
The Connecticut Light and Power Company
Operating Leases
2021	0.2
2022	0.1
2023	0.0
2024	0.0
2025	0.0
Thereafter	0.0
Future lease payments	0.3
Less amount representing interest	0.0
Present value of future minimum lease payments	0.3	0.7
Finance Leases
2021	1.5
2022	0.0
2023	0.0
2024	0.0
2025	0.0
Thereafter	0.0
Future lease payments	1.5
Less amount representing interest	0.1
Present value of future minimum lease payments	1.4	3.0
NSTAR Electric Company
Operating Leases
2021	1.6
2022	1.6
2023	1.6
2024	1.7
2025	1.7
Thereafter	27.0
Future lease payments	35.2
Less amount representing interest	11.6
Present value of future minimum lease payments	23.6	24.2
Finance Leases
2021	0.6
2022	0.6
2023	0.6
2024	0.7
2025	0.7
Thereafter	13.0
Future lease payments	16.2
Less amount representing interest	11.4
Present value of future minimum lease payments	4.8	4.4
Public Service Company of New Hampshire
Operating Leases
2021	0.1
2022	0.1
2023	0.1
2024	0.0
2025	0.0
Thereafter	0.1
Future lease payments	0.4
Less amount representing interest	0.1
Present value of future minimum lease payments	0.3	0.4
Finance Leases
2021	0.1
2022	0.1
2023	0.1
2024	0.1
2025	0.1
Thereafter	0.4
Future lease payments	0.9
Less amount representing interest	0.1
Present value of future minimum lease payments	$ 0.8	$ 0.9