SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 95,363	$ 115,285	$ 187,706	$ 206,315
Pension, SERP and PBOP Non-Service Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other Income, Net	35,500	30,000	68,100	57,800
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	25,600	23,700	49,500	50,500
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	4,800	23,200	10,800	31,000
Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other Income, Net	(2,000)	1,100	(4,000)	300
Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	30,800	37,100	64,800	67,100
Other
Summary of Accounting Policies [Line Items]
Other Income, Net	700	200	(1,500)	(400)
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	12,498	22,977	30,096	39,445
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	7,200	17,600	13,700
CL&P \| AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	1,500	5,500	5,100	11,500
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)/Income
Summary of Accounting Policies [Line Items]
Other Income, Net	(900)	(100)	(800)	(900)
CL&P \| Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	2,500	10,400	8,100	15,100
CL&P \| Other
Summary of Accounting Policies [Line Items]
Other Income, Net	100	0	100	0
NSTAR Electric
Summary of Accounting Policies [Line Items]
Other Income, Net	47,715	48,421	94,009	95,536
NSTAR Electric \| Pension, SERP and PBOP Non-Service Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other Income, Net	15,400	13,300	30,200	25,600
NSTAR Electric \| AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	14,700	15,200	28,800	31,400
NSTAR Electric \| Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	200	200	400	400
NSTAR Electric \| Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other Income, Net	(200)	500	(2,000)	900
NSTAR Electric \| Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	17,600	19,200	36,600	37,200
NSTAR Electric \| Other
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	0	0
PSNH
Summary of Accounting Policies [Line Items]
Other Income, Net	13,725	7,772	24,484	14,822
PSNH \| Pension, SERP and PBOP Non-Service Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other Income, Net	4,600	3,700	8,700	7,300
PSNH \| AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	4,500	1,700	8,400	3,600
PSNH \| Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	0	0
PSNH \| Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other Income, Net	(400)	0	(300)	(200)
PSNH \| Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	5,000	2,400	7,700	4,100
PSNH \| Other
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 0	$ 0	$ 0	$ 0