SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Summary of Accounting Policies [Line Items]
Other income, net	$ 43,768	$ 29,218	$ 124,588	$ 83,565
Pension, SERP and PBOP Non-Service   Income Components
Summary of Accounting Policies [Line Items]
Other income, net	21,700	11,000	63,900	34,300
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	10,300	10,300	28,700	31,800
Equity in Earnings of Unconsolidated Affiliates (1)
Summary of Accounting Policies [Line Items]
Other income, net	4,900	3,900	13,300	13,800
Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	(600)	1,000	700	(1,500)
Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	7,300	1,500	17,200	3,300
Gain on Sale of Property
Summary of Accounting Policies [Line Items]
Other income, net	0	1,500	100	1,800
Other
Summary of Accounting Policies [Line Items]
Other income, net	200	0	700	100
Equity In Earnings
Summary of Accounting Policies [Line Items]
Unrealized gain associated with investment			2,100	2,400
CL&P
Summary of Accounting Policies [Line Items]
Other income, net	6,903	5,917	21,690	16,273
CL&P | Pension, SERP and PBOP Non-Service   Income Components
Summary of Accounting Policies [Line Items]
Other income, net	4,200	900	11,200	3,000
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	1,700	3,500	5,100	11,400
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	(300)	1,300	1,200	0
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	1,300	200	4,100	1,800
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	100	100
NSTAR Electric
Summary of Accounting Policies [Line Items]
Other income, net	20,215	12,802	58,941	38,152
NSTAR Electric | Pension, SERP and PBOP Non-Service   Income Components
Summary of Accounting Policies [Line Items]
Other income, net	10,100	7,200	30,300	22,200
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	6,200	5,500	18,600	15,800
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates (1)
Summary of Accounting Policies [Line Items]
Other income, net	100	100	300	400
NSTAR Electric | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	(200)	(200)	600	(1,500)
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	3,900	100	8,800	700
NSTAR Electric | Gain on Sale of Property
Summary of Accounting Policies [Line Items]
Other income, net	0	0	0	300
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	3,171	4,065	11,598	10,882
PSNH | Pension, SERP and PBOP Non-Service   Income Components
Summary of Accounting Policies [Line Items]
Other income, net	2,500	1,800	7,900	5,200
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	300	900	1,200	3,500
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)	200	300	100
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	400	1,200	2,000	2,000
PSNH | Gain on Sale of Property
Summary of Accounting Policies [Line Items]
Other income, net	0	0	100	0
PSNH | Other
Summary of Accounting Policies [Line Items]
Other income, net	$ 100	$ 0	$ 100	$ 100