SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	$ 212,700	$ 195,700
Uncollectible Hardship	131,500	122,500
Bad debt expense associated with accounts receivable	$ 61,337	44,453	$ 69,466
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	$ 88,000	78,900
Uncollectible Hardship	71,900	65,500
Bad debt expense associated with accounts receivable	15,831	5,312	17,572
NSTAR Electric Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	74,500	69,700
Uncollectible Hardship	42,500	40,300
Bad debt expense associated with accounts receivable	22,279	21,252	31,728
Public Service Company Of New Hampshire
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Provision for Uncollectible Accounts	11,100	10,500
Uncollectible Hardship	0	0
Bad debt expense associated with accounts receivable	$ 6,383	$ 6,704	$ 7,288