REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Millions	May 01, 2023	Sep. 30, 2023	Dec. 31, 2022
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved		$ 251.4	$ 210.8
Regulatory costs		85.6	64.0
Regulatory assets		5,856.6	5,578.3
Operation, maintenance and vegetation expenses
Regulatory Assets [Line Items]
Purchase price of regulatory assets	$ 23.3
Regulatory assets	$ 16.9
COVID-19
Regulatory Assets [Line Items]
Regulatory costs		29.8	29.8
CL&P
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved		159.8	135.9
Regulatory costs		65.6	52.8
Regulatory assets		1,982.6	1,907.8
CL&P | COVID-19
Regulatory Assets [Line Items]
Regulatory costs		11.8	11.8
NSTAR Electric
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved		26.7	19.8
Regulatory costs		9.6	3.5
Regulatory assets		1,828.6	1,714.4
NSTAR Electric | COVID-19
Regulatory Assets [Line Items]
Regulatory costs		2.2	2.2
PSNH
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved		1.2	1.0
Regulatory assets		$ 916.5	$ 696.2