SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Summary of Accounting Policies [Line Items]
Other income, net	$ 89,831	$ 43,768	$ 255,253	$ 124,588
Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	55,300	21,700	164,500	63,900
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	12,600	10,300	33,700	28,700
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	3,800	4,900	20,800	13,300
Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	1,300	(600)	2,400	700
Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	14,100	7,300	30,300	17,200
Gain on Sale of Property
Summary of Accounting Policies [Line Items]
Other income, net	2,500	0	2,700	100
Other
Summary of Accounting Policies [Line Items]
Other income, net	200	200	900	700
Equity In Earnings
Summary of Accounting Policies [Line Items]
Unrealized gain associated with investment			12,200	2,100
CL&P
Summary of Accounting Policies [Line Items]
Other income, net	21,927	6,903	61,290	21,690
CL&P | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	16,200	4,200	48,300	11,200
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	3,700	1,700	9,300	5,100
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	200	(300)	(900)	1,200
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	1,800	1,300	4,600	4,100
CL&P | Gain on Sale of Property
Summary of Accounting Policies [Line Items]
Other income, net	0	0	0	0
CL&P | Other
Summary of Accounting Policies [Line Items]
Other income, net	0	0	0	100
NSTAR Electric
Summary of Accounting Policies [Line Items]
Other income, net	37,895	20,215	101,385	58,941
NSTAR Electric | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	21,600	10,100	64,000	30,300
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	6,500	6,200	17,500	18,600
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	100	100	200	300
NSTAR Electric | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	400	(200)	600	600
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	9,200	3,900	18,800	8,800
NSTAR Electric | Gain on Sale of Property
Summary of Accounting Policies [Line Items]
Other income, net	0	0	0	0
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other income, net	100	100	300	300
PSNH
Summary of Accounting Policies [Line Items]
Other income, net	8,073	3,171	23,365	11,598
PSNH | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	6,800	2,500	20,100	7,900
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	700	300	1,700	1,200
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	200	(100)	500	300
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	400	400	1,100	2,000
PSNH | Gain on Sale of Property
Summary of Accounting Policies [Line Items]
Other income, net	0	0	0	100
PSNH | Other
Summary of Accounting Policies [Line Items]
Other income, net	$ 0	$ 100	$ 0	$ 100