SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	$ 225,500		$ 225,500		$ 212,700
Provision for Uncollectible Hardship Accounts	146,500		146,500		131,500
Bad debt expense associated with accounts receivable	14,900	$ 21,500	46,417	$ 50,720
CL&P
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	94,100		94,100		88,000
Provision for Uncollectible Hardship Accounts	77,800		77,800		71,900
Bad debt expense associated with accounts receivable	4,500	4,400	12,084	12,135
NSTAR Electric
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	80,700		80,700		74,500
Provision for Uncollectible Hardship Accounts	49,400		49,400		42,500
Bad debt expense associated with accounts receivable	7,000	9,100	18,598	20,433
PSNH
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	10,600		10,600		11,100
Provision for Uncollectible Hardship Accounts	0		0		$ 0
Bad debt expense associated with accounts receivable	$ 1,800	$ 1,600	$ 4,977	$ 4,865