REGULATORY ACCOUNTING - Regulatory Tracker Mechanisms (Details) - Regulatory Tracker Deferrals Regulatory Assets - USD ($) $ in Millions	Feb. 01, 2018	Dec. 31, 2017
The Connecticut Light And Power Company
Regulatory Assets [Line Items]
Decoupling mechanisms permitted in recovery of an annual base amount		$ 1,059.0
NSTAR Electric Company
Regulatory Assets [Line Items]
Decoupling mechanisms permitted in recovery of an annual base amount		$ 132.4
Subsequent Event | The Connecticut Light And Power Company
Regulatory Assets [Line Items]
Decoupling mechanisms permitted in recovery of an annual base amount	$ 975.0