LEASES - Future Minimum Rental Payments under Operating Leases (Details) $ in Millions	Dec. 31, 2018 USD ($)
Operating Leased Assets [Line Items]
2019	$ 11.5
2020	9.8
2021	8.7
2022	7.2
2023	4.7
Thereafter	32.7
Future minimum lease payments	74.6
CL&P
Operating Leased Assets [Line Items]
2019	1.5
2020	1.4
2021	1.2
2022	1.1
2023	0.5
Thereafter	0.2
Future minimum lease payments	5.9
NSTAR Electric
Operating Leased Assets [Line Items]
2019	7.2
2020	6.0
2021	5.3
2022	4.4
2023	3.1
Thereafter	29.5
Future minimum lease payments	55.5
PSNH
Operating Leased Assets [Line Items]
2019	0.5
2020	0.4
2021	0.4
2022	0.4
2023	0.2
Thereafter	0.3
Future minimum lease payments	$ 2.2