SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	$ 218,100		$ 218,100		$ 195,700
Uncollectible Hardship	132,600		132,600		122,500
Bad debt expense associated with accounts receivable	21,500	$ 14,400	$ 50,720	$ 30,111
The Connecticut Light and Power Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Period of accounts receivable recoverable under financial or medical duress			180 days
Total Provision for Uncollectible Accounts	84,600		$ 84,600		78,900
Uncollectible Hardship	68,300		68,300		65,500
Bad debt expense associated with accounts receivable	4,400	3,600	$ 12,135	1,755
Yankee Gas Services Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Period of accounts receivable recoverable under financial or medical duress			90 days
NSTAR Electric Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	82,000		$ 82,000		69,700
Uncollectible Hardship	46,500		46,500		40,300
Bad debt expense associated with accounts receivable	9,100	8,100	20,433	14,937
Public Service Company of New Hampshire
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	11,300		11,300		10,500
Uncollectible Hardship	0		0		$ 0
Bad debt expense associated with accounts receivable	$ 1,600	$ 1,500	$ 4,900	$ 5,200