SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Summary of Accounting Policies [Line Items]
Other income, net	$ 34,201	$ 24,104
Pension, SERP and PBOP Non-Service   Income Components
Summary of Accounting Policies [Line Items]
Other income, net	20,100	12,700
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	9,200	10,600
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	3,700	4,000
Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	(600)	(4,300)
Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	1,500	700
Other
Summary of Accounting Policies [Line Items]
Other income, net	300	400
CL&P
Summary of Accounting Policies [Line Items]
Other income, net	4,908	1,898
CL&P | Pension, SERP and PBOP Non-Service   Income Components
Summary of Accounting Policies [Line Items]
Other income, net	2,700	1,100
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	1,700	3,900
CL&P | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	0	0
CL&P | Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	400	(3,400)
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	100	300
CL&P | Other
Summary of Accounting Policies [Line Items]
Other income, net	0	0
NSTAR Electric
Summary of Accounting Policies [Line Items]
Other income, net	16,812	12,238
NSTAR Electric | Pension, SERP and PBOP Non-Service   Income Components
Summary of Accounting Policies [Line Items]
Other income, net	10,100	7,900
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	6,200	5,000
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	100	100
NSTAR Electric | Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	200	(1,300)
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	100	100
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other income, net	100	400
PSNH
Summary of Accounting Policies [Line Items]
Other income, net	4,166	3,190
PSNH | Pension, SERP and PBOP Non-Service   Income Components
Summary of Accounting Policies [Line Items]
Other income, net	2,700	1,900
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	600	1,500
PSNH | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	0	0
PSNH | Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	100	(500)
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	700	300
PSNH | Other
Summary of Accounting Policies [Line Items]
Other income, net	$ 100	$ 0