REGULATORY ACCOUNTING - Regulatory Costs in Other Long-Term Assets (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 103.0	$ 86.3
The Connecticut Light And Power Company [Member]
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	3.5	5.9
NSTAR Electric Company [Member]
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	40.1	35.0
Public Service Company Of New Hampshire [Member]
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	16.4	8.2
Western Massachusetts Electric Company [Member]
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 24.3	$ 20.1