REGULATORY ACCOUNTING - Regulatory Costs in Other Long-Term Assets (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 108.7	$ 86.3
The Connecticut Light And Power Company
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	3.9	5.9
NSTAR Electric Company
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	42.3	35.0
Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	18.5	8.2
Western Massachusetts Electric Company
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 25.7	$ 20.1