SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Summary of Accounting Policies [Line Items]
Other income, net	$ 93,861	$ 46,619	$ 165,422	$ 80,820
Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	54,900	22,100	109,200	42,200
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	11,200	9,200	21,100	18,400
Equity in Earnings of Unconsolidated Affiliates (1)
Summary of Accounting Policies [Line Items]
Other income, net	16,600	4,700	17,100	8,400
Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	1,300	1,900	1,100	1,300
Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	9,500	8,400	16,100	9,900
Other
Summary of Accounting Policies [Line Items]
Other income, net	400	300	800	600
Equity In Earnings
Summary of Accounting Policies [Line Items]
Unrealized gain associated with investment	12,200	2,100	12,200	2,100
CL&P
Summary of Accounting Policies [Line Items]
Other income, net	19,800	9,879	39,363	14,787
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	4,300	32,100	7,000
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	2,800	1,700	5,600	3,400
CL&P | Equity in Earnings of Unconsolidated Affiliates (1)
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	(700)	1,200	(1,100)	1,500
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	1,500	2,700	2,800	2,800
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	34,259	21,915	63,490	38,727
NSTAR Electric | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	21,400	10,200	42,400	20,200
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	6,100	6,200	11,000	12,400
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates (1)
Summary of Accounting Policies [Line Items]
Other income, net	100	100	100	200
NSTAR Electric | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	500	600	200	800
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	6,100	4,800	9,600	4,900
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other income, net	100	0	200	200
PSNH
Summary of Accounting Policies [Line Items]
Other income, net	7,782	4,260	15,292	8,427
PSNH | Pension, SERP and PBOP Non-Service   Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other income, net	6,700	2,700	13,300	5,400
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	600	300	1,000	900
PSNH | Equity in Earnings of Unconsolidated Affiliates (1)
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	100	300	300	400
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	400	900	700	1,600
PSNH | Other
Summary of Accounting Policies [Line Items]
Other income, net	$ 0	$ 100	$ 0	$ 100