REGULATORY ACCOUNTING REGULATORY ACCOUNTING - Regulatory Costs in Long-Term Assets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Amount of Regulatory Costs Not yet Approved	$ 122.9	$ 105.8
The Connecticut Light And Power Company
Amount of Regulatory Costs Not yet Approved	42.1	18.2
NSTAR Electric Company
Amount of Regulatory Costs Not yet Approved	49.3	42.7
Public Service Company Of New Hampshire
Amount of Regulatory Costs Not yet Approved	$ 12.2	$ 27.2