REGULATORY ACCOUNTING - Regulatory Costs in Other Long-Term Assets (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 86.3	$ 75.3
The Connecticut Light And Power Company [Member]
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	5.9	3.1
NSTAR Electric Company [Member]
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	35.0	35.4
Public Service Company Of New Hampshire [Member]
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	8.2	4.8
Western Massachusetts Electric Company [Member]
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 20.1	$ 16.7