SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	$ 66,773	$ 106,599
Cash and Restricted Cash as reported on the Statements of Cash Flows	221,008	264,950	$ 117,063	$ 209,324
The Connecticut Light and Power Company
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	55,804	90,801
Cash and Restricted Cash as reported on the Statements of Cash Flows	74,788	99,809	4,971	91,613
NSTAR Electric Company
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	745	102
Cash and Restricted Cash as reported on the Statements of Cash Flows	18,179	17,410	6,312	14,659
Public Service Company of New Hampshire
Restricted Cash and Cash Equivalents Items [Line Items]
Cash as reported on the Balance Sheets	15	141
Cash and Restricted Cash as reported on the Statements of Cash Flows	35,126	39,555	$ 36,688	$ 52,723
Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	78,200	73,600
Special Deposits | The Connecticut Light and Power Company
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	18,700	8,700
Special Deposits | NSTAR Electric Company
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	17,400	17,200
Special Deposits | Public Service Company of New Hampshire
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	31,400	36,800
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	31,300	41,200
Marketable Securities | The Connecticut Light and Power Company
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	300	300
Marketable Securities | NSTAR Electric Company
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	100	100
Marketable Securities | Public Service Company of New Hampshire
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	500	600
Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	44,700	43,600
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		$ 2,100