ACCUMULATED OTHER COMPREHENSIVE INCOME/(LOSS) - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Related tax effect recognized in AOCI	$ 0.2	$ 4.1	$ 4.0
Pre-tax amount reclassified from AOCI as a decrease to net income	2.5
Pre-tax amount reclassified from AOCI as a decrease to net income as a result of amortization of Pension, SERP and PBOP costs	6.3
NSTAR Gas
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Pre-tax amount reclassified from AOCI as a decrease to net income	0.7
Public Service Company Of New Hampshire
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Pre-tax amount reclassified from AOCI as a decrease to net income	$ 1.8