SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	$ 224,800	$ 212,700
Provision for Uncollectible Hardship Accounts	143,300	131,500
Uncollectible Expense	63,446	61,337	$ 44,453
CL&P
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	97,300	88,000
Provision for Uncollectible Hardship Accounts	80,100	71,900
Uncollectible Expense	15,948	15,831	5,312
NSTAR Electric
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	75,400	74,500
Provision for Uncollectible Hardship Accounts	43,900	42,500
Uncollectible Expense	25,079	22,279	21,252
PSNH
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	10,500	11,100
Provision for Uncollectible Hardship Accounts	0	0
Uncollectible Expense	$ 6,726	$ 6,383	$ 6,704