SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Operating Expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summary of Accounting Policies [Line Items]
Fuel power costs	$ 372.2	$ 516.7	$ 599.4
Public Service Company Of New Hampshire [Member]
Summary of Accounting Policies [Line Items]
Fuel power costs	$ 45.0	$ 85.4	$ 113.4