SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Operating Expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Summary of Accounting Policies [Line Items]
Natural gas and fuel costs	$ 462.1	$ 442.6	$ 432.5
PSNH
Summary of Accounting Policies [Line Items]
Natural gas and fuel costs	$ 0.0	$ 7.9	$ 43.4