SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	$ 106,599	$ 15,432
Cash and Restricted Cash reported on the Statements of Cash Flows	264,950	117,063	$ 209,324	$ 85,890
The Connecticut Light and Power Company
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	90,801	0
Cash and Restricted Cash reported on the Statements of Cash Flows	99,809	4,971	91,613	9,619
NSTAR Electric Company
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	102	52
Cash and Restricted Cash reported on the Statements of Cash Flows	17,410	6,312	14,659	14,708
Public Service Company of New Hampshire
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	141	413
Cash and Restricted Cash reported on the Statements of Cash Flows	39,555	36,688	$ 52,723	$ 2,191
Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	73,600	52,500
Special Deposits | The Connecticut Light and Power Company
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	8,700	4,600
Special Deposits | NSTAR Electric Company
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	17,200	6,200
Special Deposits | Public Service Company of New Hampshire
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	36,800	32,500
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	41,200	46,000
Marketable Securities | The Connecticut Light and Power Company
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	300	400
Marketable Securities | NSTAR Electric Company
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	100	0
Marketable Securities | Public Service Company of New Hampshire
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	43,600	3,200
Other Long-Term Assets | The Connecticut Light and Power Company
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
Other Long-Term Assets | NSTAR Electric Company
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 0	0
Other Long-Term Assets | Public Service Company of New Hampshire
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash		$ 3,200