SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Supplemental Cash Flow (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	$ 172.0	$ 186.3
The Connecticut Light And Power Company [Member]
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	85.9	59.5
NSTAR Electric Company [Member]
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	29.5	38.5
Public Service Company Of New Hampshire [Member]
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	20.2	31.1
Western Massachusetts Electric Company [Member]
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	$ 10.6	$ 14.6