SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Supplemental Cash Flow (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	$ 307.7	$ 203.6
The Connecticut Light And Power Company
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	113.4	64.5
NSTAR Electric Company
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	55.4	39.4
Public Service Company Of New Hampshire
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	39.6	31.0
Western Massachusetts Electric Company
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	$ 37.1	$ 17.6