SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Supplemental Cash Flow (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	$ 274.4	$ 220.5
The Connecticut Light And Power Company
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	117.7	104.2
NSTAR Electric Company
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	59.5	49.4
Public Service Company Of New Hampshire
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	$ 36.0	$ 28.7