REGULATORY ACCOUNTING - Regulatory Costs in Other Long-Term Assets (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 123.3	$ 105.8
Regulatory assets	5,338.7	5,239.3
The Connecticut Light And Power Company
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	21.4	18.2
Regulatory assets	1,729.3	1,645.2
NSTAR Electric Company
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	43.6	42.7
Regulatory assets	1,564.3	1,524.5
Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	38.7	27.2
Regulatory assets	938.7	$ 940.8
Deferred Costs from Generation Asset Sale
Regulatory Assets [Line Items]
Regulatory assets	156.0
Deferred Costs from Generation Asset Sale | The Connecticut Light And Power Company
Regulatory Assets [Line Items]
Regulatory assets	52.0
Deferred Costs from Generation Asset Sale | NSTAR Electric Company
Regulatory Assets [Line Items]
Regulatory assets	100.0
Deferred Costs from Generation Asset Sale | Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Regulatory assets	$ 4.0