SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	$ 303,810	$ 38,165
Restricted cash included in:
Prepayments and Other Current Assets	35,500	24,400
Marketable Securities	23,300	23,300
Cash, Cash Equivalents, and Restricted Cash reported on the Statements of Cash Flows	362,615	85,890	$ 126,285	$ 106,750
The Connecticut Light And Power Company
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	240,217	6,028
Restricted cash included in:
Prepayments and Other Current Assets	3,100	3,100
Marketable Securities	500	500
Cash, Cash Equivalents, and Restricted Cash reported on the Statements of Cash Flows	243,839	9,619	18,930	8,403
NSTAR Electric Company
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	9,471	1,763
Restricted cash included in:
Prepayments and Other Current Assets	12,900	12,800
Marketable Securities	100	100
Cash, Cash Equivalents, and Restricted Cash reported on the Statements of Cash Flows	22,465	14,708	16,065	15,506
Public Service Company Of New Hampshire
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	2,917	900
Restricted cash included in:
Prepayments and Other Current Assets	11,600	500
Marketable Securities	800	800
Cash, Cash Equivalents, and Restricted Cash reported on the Statements of Cash Flows	$ 15,253	$ 2,191	$ 9,116	$ 5,953