SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Summary of Accounting Policies [Line Items]
Other income, net	$ 94,875	$ 93,861	$ 183,857	$ 165,422
Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	33,300	54,900	68,100	109,200
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	19,000	11,200	34,500	21,100
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	5,000	16,600	8,800	17,100
Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	(1,400)	1,300	(3,100)	1,100
Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	21,400	9,500	44,500	16,100
Other
Summary of Accounting Policies [Line Items]
Other income, net	17,600	400	31,100	800
Equity in Earnings
Summary of Accounting Policies [Line Items]
Unrealized gain associated with investment		12,200		12,200
CL&P
Summary of Accounting Policies [Line Items]
Other income, net	13,362	19,800	28,299	39,363
CL&P | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	8,400	16,200	17,900	32,100
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	4,600	2,800	8,700	5,600
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	(1,100)	(700)	(1,700)	(1,100)
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	1,500	1,500	3,400	2,800
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	40,909	34,259	80,782	63,490
NSTAR Electric | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	14,100	21,400	28,800	42,400
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	12,000	6,100	21,500	11,000
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	100	100	200	100
NSTAR Electric | Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	300	500	(200)	200
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	14,400	6,100	30,400	9,600
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other income, net	0	100	100	200
PSNH
Summary of Accounting Policies [Line Items]
Other income, net	6,301	7,782	12,019	15,292
PSNH | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	3,900	6,700	8,100	13,300
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	1,100	600	1,800	1,000
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)	100	(400)	300
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	1,200	400	2,100	700
PSNH | Other
Summary of Accounting Policies [Line Items]
Other income, net	$ 400	$ 0	$ 400	$ 0