SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Supplemental Cash Flow (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	$ 303.7	$ 307.7
The Connecticut Light and Power Company
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	103.0	113.4
NSTAR Electric Company
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	62.5	92.5
Public Service Company of New Hampshire
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	$ 48.3	$ 39.6