SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Cash	$ 15,432	$ 108,068
Cash and Restricted Cash reported on the Statements of Cash Flows	117,063	209,324	$ 85,890	$ 106,750
CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Cash	0	87,721
Cash and Restricted Cash reported on the Statements of Cash Flows	4,971	91,613	9,619	8,403
NSTAR Electric
Restricted Cash and Cash Equivalents Items [Line Items]
Cash	52	1,606
Cash and Restricted Cash reported on the Statements of Cash Flows	6,312	14,659	14,708	15,507
PSNH
Restricted Cash and Cash Equivalents Items [Line Items]
Cash	413	1,439
Cash and Restricted Cash reported on the Statements of Cash Flows	36,688	52,723	$ 2,191	$ 5,953
Prepayments and Other Current Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	52,500	72,100
Prepayments and Other Current Assets | CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	4,600	3,500
Prepayments and Other Current Assets | NSTAR Electric
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	6,200	13,000
Prepayments and Other Current Assets | PSNH
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash		47,500
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	46,000	25,900
Marketable Securities | CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	400	400
Marketable Securities | NSTAR Electric
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	100
Marketable Securities | PSNH
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	600	600
Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	3,200	3,200
Other Long-Term Assets | CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
Other Long-Term Assets | NSTAR Electric
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 0	0
Other Long-Term Assets | PSNH
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash		$ 3,200