SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	$ 35,145	$ 108,068
Cash and Restricted Cash reported on the Statements of Cash Flows	108,372	209,324	$ 362,615	$ 85,890
Prepayments and Other Current Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	45,900	72,100
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	24,200	25,900
Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	3,200	3,200
CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	7,496	87,721
Cash and Restricted Cash reported on the Statements of Cash Flows	11,404	91,613	243,839	9,619
CL&P | Prepayments and Other Current Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	3,600	3,500
CL&P | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	300	400
CL&P | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
NSTAR Electric
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	1,769	1,606
Cash and Restricted Cash reported on the Statements of Cash Flows	14,905	14,659	22,465	14,708
NSTAR Electric | Prepayments and Other Current Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	13,000	13,000
NSTAR Electric | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	100	100
NSTAR Electric | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
PSNH
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	3,961	1,439
Cash and Restricted Cash reported on the Statements of Cash Flows	28,796	52,723	$ 15,253	$ 2,191
PSNH | Prepayments and Other Current Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	21,100	47,500
PSNH | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	500	600
PSNH | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 3,200	$ 3,200