SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 85,830	$ 112,555	$ 273,536	$ 318,870
Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other Income, Net	35,600	31,600	103,700	89,400
AFUDC Equity (1)
Summary of Accounting Policies [Line Items]
Other Income, Net	19,700	22,700	69,200	73,200
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	4,500	17,100	15,300	48,100
Investment Loss
Summary of Accounting Policies [Line Items]
Other Income, Net	(2,800)	(2,100)	(6,800)	(1,800)
Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	29,400	38,900	94,200	106,000
Other
Summary of Accounting Policies [Line Items]
Other Income, Net	(600)	4,400	(2,100)	4,000
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	9,991	21,944	40,087	61,389
CL&P | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other Income, Net	9,300	7,700	26,900	21,400
CL&P | AFUDC Equity (1)
Summary of Accounting Policies [Line Items]
Other Income, Net	(1,200)	5,400	3,900	16,900
CL&P | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	0	0
CL&P | Investment Loss
Summary of Accounting Policies [Line Items]
Other Income, Net	(1,000)	(900)	(1,800)	(1,700)
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	2,900	9,700	11,000	24,800
CL&P | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	100	0
NSTAR Electric
Summary of Accounting Policies [Line Items]
Other Income, Net	47,074	47,489	141,082	143,024
NSTAR Electric | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other Income, Net	15,600	13,900	45,800	39,500
NSTAR Electric | AFUDC Equity (1)
Summary of Accounting Policies [Line Items]
Other Income, Net	14,300	13,000	43,100	44,400
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	200	200	600	600
NSTAR Electric | Investment Loss
Summary of Accounting Policies [Line Items]
Other Income, Net	(700)	(200)	(2,700)	700
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	17,600	20,600	54,200	57,800
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	100	0	100	0
PSNH
Summary of Accounting Policies [Line Items]
Other Income, Net	9,032	8,097	33,515	22,919
PSNH | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other Income, Net	4,500	3,900	13,200	11,200
PSNH | AFUDC Equity (1)
Summary of Accounting Policies [Line Items]
Other Income, Net	2,200	1,500	10,600	5,100
PSNH | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	0	0
PSNH | Investment Loss
Summary of Accounting Policies [Line Items]
Other Income, Net	(400)	(300)	(700)	(500)
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	2,700	3,000	10,400	7,100
PSNH | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 0	$ 0	$ 0	$ 0