SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Summary of Accounting Policies [Line Items]
Other income, net	$ 115,285	$ 94,875	$ 206,315	$ 183,857
Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	30,000	33,300	57,800	68,100
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	23,700	19,000	50,500	34,500
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	23,200	5,000	31,000	8,800
Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	1,100	(1,400)	300	(3,100)
Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	37,100	21,400	67,100	44,500
Other
Summary of Accounting Policies [Line Items]
Other income, net	200	17,600	(400)	31,100
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	22,977	13,362	39,445	28,299
CL&P | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	7,200	8,400	13,700	17,900
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	5,500	4,600	11,500	8,700
CL&P | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	0	0	0	0
CL&P | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	(100)	(1,100)	(900)	(1,700)
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	10,400	1,500	15,100	3,400
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	48,421	40,909	95,536	80,782
NSTAR Electric | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	13,300	14,100	25,600	28,800
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	15,200	12,000	31,400	21,500
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	200	100	400	200
NSTAR Electric | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	500	300	900	(200)
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	19,200	14,400	37,200	30,400
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other income, net	0	0	0	100
PSNH
Summary of Accounting Policies [Line Items]
Other income, net	7,772	6,301	14,822	12,019
PSNH | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	3,700	3,900	7,300	8,100
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	1,700	1,100	3,600	1,800
PSNH | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	0	0	0	0
PSNH | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	0	(300)	(200)	(400)
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	2,400	1,200	4,100	2,100
PSNH | Other
Summary of Accounting Policies [Line Items]
Other income, net	$ 0	$ 400	$ 0	$ 400