SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Supplemental Cash Flow (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	$ 203.6	$ 160.7
The Connecticut Light And Power Company [Member]
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	64.5	46.0
NSTAR Electric Company [Member]
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	39.4	31.2
Public Service Company Of New Hampshire [Member]
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	31.0	33.8
Western Massachusetts Electric Company [Member]
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	$ 17.6	$ 15.5