SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	$ 59,092	$ 38,165
Cash, Cash Equivalents, and Restricted Cash reported on the Statements of Cash Flows	134,605	85,890	$ 171,976	$ 106,750
Prepayments and Other Current Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	51,400	24,400
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	20,900	23,300
Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	3,200	0
The Connecticut Light and Power Company
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	977	6,028
Cash, Cash Equivalents, and Restricted Cash reported on the Statements of Cash Flows	4,880	9,619	12,916	8,403
The Connecticut Light and Power Company | Prepayments and Other Current Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	3,500	3,100
The Connecticut Light and Power Company | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	400	500
The Connecticut Light and Power Company | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
NSTAR Electric Company
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	2,367	1,763
Cash, Cash Equivalents, and Restricted Cash reported on the Statements of Cash Flows	15,361	14,708	102,813	15,506
NSTAR Electric Company | Prepayments and Other Current Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	12,900	12,800
NSTAR Electric Company | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	100	100
NSTAR Electric Company | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
Public Service Company of New Hampshire
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	7,462	900
Cash, Cash Equivalents, and Restricted Cash reported on the Statements of Cash Flows	38,166	2,191	$ 1,834	$ 5,953
Public Service Company of New Hampshire | Prepayments and Other Current Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	26,800	500
Public Service Company of New Hampshire | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	700	800
Public Service Company of New Hampshire | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 3,200	$ 0