SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 101,159	$ 92,344
Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other Income, Net	44,200	32,600
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	23,900	23,900
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	3,800	6,000
Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	300	(2,000)
Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	28,400	34,000
Other
Summary of Accounting Policies [Line Items]
Other Income, Net	600	(2,200)
CL&P
Summary of Accounting Policies [Line Items]
Other Income, Net	17,616	17,599
CL&P | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other Income, Net	11,900	8,300
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	4,600	3,600
CL&P | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0
CL&P | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	(100)	100
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	1,200	5,600
CL&P | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0
NSTAR Electric
Summary of Accounting Policies [Line Items]
Other Income, Net	51,684	46,292
NSTAR Electric | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other Income, Net	19,000	14,800
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	15,700	14,100
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	100	200
NSTAR Electric | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	400	(1,800)
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	16,400	19,000
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	100	0
PSNH
Summary of Accounting Policies [Line Items]
Other Income, Net	10,502	10,758
PSNH | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other Income, Net	5,200	4,100
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	2,300	3,900
PSNH | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0
PSNH | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net	0	100
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	3,000	2,700
PSNH | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 0	$ 0