SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Taxes (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accounting Policies [Abstract]
Expenses recorded as taxes other than income taxes	$ 12,700,000		$ 25.4
Schedule of Gross Tax Receipts [Line Items]
Excise and sales taxes	35,600,000	$ 35,700,000	79,000,000	$ 77,900,000
The Connecticut Light and Power Company
Schedule of Gross Tax Receipts [Line Items]
Excise and sales taxes	$ 31,500,000	$ 31,800,000	$ 67,100,000	$ 65,700,000