DIVIDEND RESTRICTIONS - Narrative (Details) $ in Millions	Dec. 31, 2017 USD ($)
Schedule of Restrictions on Retained Earnings [Line Items]
Consolidated total debt to total capital ratio (as a percentage)	0.65
Retained earnings subject to restrictions	$ 3,600.0
The Connecticut Light And Power Company
Schedule of Restrictions on Retained Earnings [Line Items]
Retained earnings subject to restrictions	1,400.0
NSTAR Electric Company
Schedule of Restrictions on Retained Earnings [Line Items]
Retained earnings subject to restrictions	1,900.0
Public Service Company Of New Hampshire
Schedule of Restrictions on Retained Earnings [Line Items]
Retained earnings subject to restrictions	511.4
Retained earnings subject to FERC hydroelectric license conditions	$ 14.3