SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Summary of Accounting Policies [Line Items]
Other income, net	$ 79,123	$ 89,831	$ 262,980	$ 255,253
Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	28,500	55,300	96,600	164,500
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	21,000	12,600	55,500	33,700
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	2,700	3,800	11,500	20,800
Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	2,700	1,300	(400)	2,400
Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	24,100	14,100	68,600	30,300
Other
Summary of Accounting Policies [Line Items]
Other income, net	100	2,700	31,200	3,600
Equity in Earnings
Summary of Accounting Policies [Line Items]
Unrealized gain associated with investment				12,200
CL&P
Summary of Accounting Policies [Line Items]
Other income, net	18,311	21,927	46,610	61,290
CL&P | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	7,000	16,200	24,900	48,300
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	5,200	3,700	13,900	9,300
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	1,500	200	(200)	(900)
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	4,600	1,800	8,000	4,600
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	39,908	37,895	120,690	101,385
NSTAR Electric | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	12,900	21,600	41,700	64,000
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	12,500	6,500	34,000	17,500
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	100	100	300	200
NSTAR Electric | Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	700	400	500	600
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	13,600	9,200	44,000	18,800
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other income, net	100	100	200	300
PSNH
Summary of Accounting Policies [Line Items]
Other income, net	7,180	8,073	19,199	23,365
PSNH | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	3,700	6,800	11,800	20,100
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	1,600	700	3,400	1,700
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	200	0	500
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	1,500	400	3,600	1,100
PSNH | Other
Summary of Accounting Policies [Line Items]
Other income, net	$ 0	$ 0	$ 400	$ 0