SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2024	Jun. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Summary of Accounting Policies [Line Items]
Other income, net	$ 112,555		$ 79,123	$ 318,870	$ 262,980
Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	31,600		28,500	89,400	96,600
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	22,700		21,000	73,200	55,500
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	17,100		2,700	48,100	11,500
Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	(2,100)		2,700	(1,800)	(400)
Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	38,900		24,100	106,000	68,600
Other
Summary of Accounting Policies [Line Items]
Other income, net	4,400		100	4,000	31,200
Equity in Earnings
Summary of Accounting Policies [Line Items]
Unrealized gain associated with investment		$ 23,400
CL&P
Summary of Accounting Policies [Line Items]
Other income, net	21,944		18,311	61,389	46,610
CL&P | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	7,700		7,000	21,400	24,900
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	5,400		5,200	16,900	13,900
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)/Income
Summary of Accounting Policies [Line Items]
Other income, net	(900)		1,500	(1,700)	(200)
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	9,700		4,600	24,800	8,000
CL&P | Other
Summary of Accounting Policies [Line Items]
Other income, net	0		0	0	0
NSTAR Electric
Summary of Accounting Policies [Line Items]
Other income, net	47,489		39,908	143,024	120,690
NSTAR Electric | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	13,900		12,900	39,500	41,700
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	13,000		12,500	44,400	34,000
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	200		100	600	300
NSTAR Electric | Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	(200)		700	700	500
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	20,600		13,600	57,800	44,000
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other income, net	0		100	0	200
PSNH
Summary of Accounting Policies [Line Items]
Other income, net	8,097		7,180	22,919	19,199
PSNH | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	3,900		3,700	11,200	11,800
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	1,500		1,600	5,100	3,400
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	(300)		400	(500)	0
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	3,000		1,500	7,100	3,600
PSNH | Other
Summary of Accounting Policies [Line Items]
Other income, net	$ 0		$ 0	$ 0	$ 400