SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Supplemental Cash Flow (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	$ 220.5	$ 125.6
The Connecticut Light And Power Company [Member]
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	104.2	52.6
NSTAR Electric Company [Member]
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	29.8	11.7
Public Service Company Of New Hampshire [Member]
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	28.7	26.8
Western Massachusetts Electric Company [Member]
Schedule of Supplemental Cash Flow [Line Items]
Capital expenditures incurred but not yet paid	$ 19.6	$ 10.7