SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Income (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2024 USD ($)	Dec. 31, 2025 USD ($)	Dec. 31, 2024 USD ($) project	Dec. 31, 2023 USD ($)
Summary of Accounting Policies [Line Items]
Other Income, Net		$ 378,854	$ 410,482	$ 348,069
Unrealized gain associated with investment		19,900	$ 51,900	15,500
Number of offshore wind projects | project			3
Pension, SERP and PBOP Non-Service Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other Income, Net		139,300	$ 115,400	132,900
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net		99,000	97,800	78,100
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net		19,900	51,900	15,500
Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other Income, Net		(5,400)	600	(4,900)
Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net		125,500	138,200	94,200
Other
Summary of Accounting Policies [Line Items]
Other Income, Net		600	6,600	32,300
Equity in Earnings Of Unconsolidated Affiliate
Summary of Accounting Policies [Line Items]
Unrealized gain associated with investment	$ 23,400
CL&P
Summary of Accounting Policies [Line Items]
Other Income, Net		59,736	77,591	61,560
CL&P | Pension, SERP and PBOP Non-Service Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other Income, Net		36,200	28,200	34,900
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net		12,200	22,400	20,000
CL&P | Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other Income, Net		(1,700)	(1,400)	(2,400)
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net		12,900	28,300	9,000
CL&P | Other
Summary of Accounting Policies [Line Items]
Other Income, Net		100	100	100
NSTAR Electric
Summary of Accounting Policies [Line Items]
Other Income, Net		192,646	191,405	164,129
NSTAR Electric | Pension, SERP and PBOP Non-Service Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other Income, Net		61,400	52,800	57,400
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net		58,400	58,800	45,700
NSTAR Electric | Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other Income, Net		(1,600)	1,900	(200)
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net		72,700	77,000	60,600
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other Income, Net		1,700	900	600
PSNH
Summary of Accounting Policies [Line Items]
Other Income, Net		42,988	31,123	26,597
PSNH | Pension, SERP and PBOP Non-Service Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other Income, Net		17,600	14,900	16,200
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net		13,000	7,000	5,400
PSNH | Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other Income, Net		(700)	(500)	(700)
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net		13,000	9,600	5,300
PSNH | Other
Summary of Accounting Policies [Line Items]
Other Income, Net		$ 100	$ 100	$ 400