ACCUMULATED OTHER COMPREHENSIVE INCOME/(LOSS) - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Related tax effect recognized in AOCI	$ (2.0)	$ 4.0	$ 22.3
Pre-tax amount reclassified from AOCI as a decrease to net income		3.4
Pre-tax amount reclassified from AOCI as a decrease to net income as a result of amortization of Pension, SERP and PBOP costs		6.6
The Connecticut Light And Power Company [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Pre-tax amount reclassified from AOCI as a decrease to net income		0.6
Public Service Company Of New Hampshire [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Pre-tax amount reclassified from AOCI as a decrease to net income		2.0
Western Massachusetts Electric Company [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Pre-tax amount reclassified from AOCI as a decrease to net income		$ 0.7