SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Other Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summary of Accounting Policies [Line Items]
Excise and sales taxes	$ 162.7	$ 147.2	$ 148.2
The Connecticut Light And Power Company [Member]
Summary of Accounting Policies [Line Items]
Excise and sales taxes	$ 145.2	$ 128.5	$ 127.9