SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Operating Expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of Accounting Policies [Line Items]
Fuel power costs	$ 432.5	$ 372.2	$ 516.7
Public Service Company Of New Hampshire
Summary of Accounting Policies [Line Items]
Fuel power costs	$ 43.4	$ 45.0	$ 85.4