SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Summary of Accounting Policies [Line Items]
Other income, net	$ 46,619	$ 30,243	$ 80,820	$ 54,347
Pension, SERP and PBOP Non-Service   Income Components
Summary of Accounting Policies [Line Items]
Other income, net	22,100	10,700	42,200	23,400
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	9,200	10,800	18,400	21,400
Equity in Earnings of Unconsolidated Affiliates (1)
Summary of Accounting Policies [Line Items]
Other income, net	4,700	5,900	8,400	9,800
Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	1,900	1,800	1,300	(2,500)
Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	8,400	1,000	9,900	1,800
Other
Summary of Accounting Policies [Line Items]
Other income, net	300	0	600	400
Equity In Earnings
Summary of Accounting Policies [Line Items]
Unrealized gain associated with investment	2,100	2,400	2,100	2,400
CL&P
Summary of Accounting Policies [Line Items]
Other income, net	9,879	8,459	14,787	10,357
CL&P | Pension, SERP and PBOP Non-Service   Income Components
Summary of Accounting Policies [Line Items]
Other income, net	4,300	900	7,000	2,100
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	1,700	3,900	3,400	7,900
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	1,200	2,200	1,500	(1,300)
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	2,700	1,400	2,800	1,600
CL&P | Other
Summary of Accounting Policies [Line Items]
Other income, net	0	100	100	100
NSTAR Electric
Summary of Accounting Policies [Line Items]
Other income, net	21,915	13,112	38,727	25,350
NSTAR Electric | Pension, SERP and PBOP Non-Service   Income Components
Summary of Accounting Policies [Line Items]
Other income, net	10,200	7,100	20,200	15,000
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	6,200	5,300	12,400	10,300
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates (1)
Summary of Accounting Policies [Line Items]
Other income, net	100	100	200	200
NSTAR Electric | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	600	0	800	(1,300)
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	4,800	500	4,900	700
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other income, net	0	100	200	500
PSNH
Summary of Accounting Policies [Line Items]
Other income, net	4,260	3,626	8,427	6,817
PSNH | Pension, SERP and PBOP Non-Service   Income Components
Summary of Accounting Policies [Line Items]
Other income, net	2,700	1,600	5,400	3,500
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	300	1,200	900	2,600
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	300	300	400	(200)
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	900	500	1,600	800
PSNH | Other
Summary of Accounting Policies [Line Items]
Other income, net	$ 100	$ 0	$ 100	$ 100