SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	$ 22,688	$ 108,068
Cash and Restricted Cash reported on the Statements of Cash Flows	151,880	209,324	$ 134,605	$ 85,890
Prepayments and Other Current Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	37,700	72,100
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	19,600	25,900
Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	71,900	3,200
CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	9,286	87,721
Cash and Restricted Cash reported on the Statements of Cash Flows	14,281	91,613	4,880	9,619
CL&P | Prepayments and Other Current Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	4,600	3,500
CL&P | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	400	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	264	1,606
Cash and Restricted Cash reported on the Statements of Cash Flows	6,508	14,659	15,361	14,708
NSTAR Electric | Prepayments and Other Current Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	6,100	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	1,647	1,439
Cash and Restricted Cash reported on the Statements of Cash Flows	24,012	52,723	$ 38,166	$ 2,191
PSNH | Prepayments and Other Current Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	18,500	47,500
PSNH | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	700	600
PSNH | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 3,200	$ 3,200