LEASES - Future Minimum Lease Payments under Long-Term Leases (Details) $ in Millions	Dec. 31, 2019 USD ($)
Operating Leases
2020	$ 10.2
2021	9.0
2022	7.4
2023	4.9
2024	2.8
Thereafter	28.9
Future lease payments	63.2
Less amount representing interest	13.3
Present value of future minimum lease payments	49.9
Finance Leases
2020	3.4
2021	2.9
2022	1.5
2023	0.8
2024	0.8
Thereafter	13.1
Future lease payments	22.5
Less amount representing interest	12.0
Present value of future minimum lease payments	10.5
CL&P
Operating Leases
2020	0.5
2021	0.2
2022	0.0
2023	0.0
2024	0.0
Thereafter	0.1
Future lease payments	0.8
Less amount representing interest	0.1
Present value of future minimum lease payments	0.7
Finance Leases
2020	2.0
2021	1.5
2022	0.0
2023	0.0
2024	0.0
Thereafter	0.0
Future lease payments	3.5
Less amount representing interest	0.5
Present value of future minimum lease payments	3.0
NSTAR Electric
Operating Leases
2020	1.8
2021	1.6
2022	1.6
2023	1.6
2024	1.7
Thereafter	28.7
Future lease payments	37.0
Less amount representing interest	12.8
Present value of future minimum lease payments	24.2
Finance Leases
2020	0.6
2021	0.6
2022	0.6
2023	0.6
2024	0.7
Thereafter	12.6
Future lease payments	15.7
Less amount representing interest	11.3
Present value of future minimum lease payments	4.4
PSNH
Operating Leases
2020	0.1
2021	0.1
2022	0.1
2023	0.0
2024	0.0
Thereafter	0.2
Future lease payments	0.5
Less amount representing interest	0.1
Present value of future minimum lease payments	0.4
Finance Leases
2020	0.1
2021	0.1
2022	0.1
2023	0.1
2024	0.1
Thereafter	0.5
Future lease payments	1.0
Less amount representing interest	0.1
Present value of future minimum lease payments	$ 0.9