SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total provision for uncollectible accounts	$ 223,300		$ 212,700
Provision for uncollectible hardship accounts	134,600		131,500
Bad debt expense associated with accounts receivable	18,565	$ 19,613
CL&P
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total provision for uncollectible accounts	93,000		88,000
Provision for uncollectible hardship accounts	75,200		71,900
Bad debt expense associated with accounts receivable	4,116	3,912
NSTAR Electric
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total provision for uncollectible accounts	74,800		74,500
Provision for uncollectible hardship accounts	40,900		42,500
Bad debt expense associated with accounts receivable	5,953	7,526
PSNH
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total provision for uncollectible accounts	11,000		11,100
Provision for uncollectible hardship accounts	0		$ 0
Bad debt expense associated with accounts receivable	$ 1,700	$ 1,700