SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	$ 226,400		$ 226,400		$ 212,700
Provision for Uncollectible Hardship Accounts	137,900		137,900		131,500
Bad debt expense associated with accounts receivable	13,000	$ 9,500	31,546	$ 29,250
CL&P
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	88,700		88,700		88,000
Provision for Uncollectible Hardship Accounts	70,900		70,900		71,900
Bad debt expense associated with accounts receivable	3,500	3,800	7,627	7,747
NSTAR Electric
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	81,800		81,800		74,500
Provision for Uncollectible Hardship Accounts	47,700		47,700		42,500
Bad debt expense associated with accounts receivable	5,700	3,800	11,605	11,301
PSNH
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	11,200		11,200		11,100
Provision for Uncollectible Hardship Accounts	0		0		$ 0
Bad debt expense associated with accounts receivable	$ 1,400	$ 1,500	$ 3,100	$ 3,200