SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Other Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of Accounting Policies [Line Items]
Excise and sales taxes	$ 157.4	$ 162.7	$ 147.2
Taxes Other than Income taxes	25.4
The Connecticut Light And Power Company
Summary of Accounting Policies [Line Items]
Excise and sales taxes	$ 137.5	$ 145.2	$ 128.5