SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Operating Expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Summary of Accounting Policies [Line Items]
Natural gas and fuel costs	$ 464.2	$ 462.1	$ 442.6
Public Service Company of New Hampshire
Summary of Accounting Policies [Line Items]
Natural gas and fuel costs	$ 0.0	$ 0.0	$ 7.9