REGULATORY ACCOUNTING - Regulatory Costs in Other Long-Term Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Regulatory assets	$ 5,239.3	$ 4,526.3
Amount of regulatory costs not yet approved	105.8	86.3
The Connecticut Light And Power Company
Regulatory Assets [Line Items]
Regulatory assets	1,645.2	1,727.1
Amount of regulatory costs not yet approved	18.2	5.9
NSTAR Electric Company
Regulatory Assets [Line Items]
Regulatory assets	1,524.5	1,538.5
Amount of regulatory costs not yet approved	42.7	55.1
Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Regulatory assets	940.8	362.7
Amount of regulatory costs not yet approved	27.2	$ 8.2
Electricity Generation Plant, Non-Nuclear | Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Deferred Costs from Generation Asset Sale	$ (516.1)