DIVIDEND RESTRICTIONS - Narrative (Details) $ in Millions	Dec. 31, 2016 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	$ 3,200.0
The Connecticut Light And Power Company [Member]
Schedule of Restrictions on Retained Earnings [Line Items]
Retained earnings subject to restrictions	1,300.0
NSTAR Electric Company [Member]
Schedule of Restrictions on Retained Earnings [Line Items]
Retained earnings subject to restrictions	1,600.0
Public Service Company Of New Hampshire [Member]
Schedule of Restrictions on Retained Earnings [Line Items]
Retained earnings subject to restrictions	549.3
Retained earnings subject to FERC hydroelectric license conditions	13.8
Western Massachusetts Electric Company [Member]
Schedule of Restrictions on Retained Earnings [Line Items]
Retained earnings subject to restrictions	$ 218.2