SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Summary of Accounting Policies [Line Items]
Other income, net	$ 24,104	$ 30,985
Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other income, net	12,700	7,400
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	10,600	10,900
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	4,000	5,000
Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	(4,300)	1,200
Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	700	6,500
Other
Summary of Accounting Policies [Line Items]
Other income, net	400	0
CL&P
Summary of Accounting Policies [Line Items]
Other income, net	1,898	3,880
CL&P | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other income, net	1,100	(600)
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	3,900	2,600
CL&P | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	0	0
CL&P | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	(3,400)	1,700
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	300	400
CL&P | Other
Summary of Accounting Policies [Line Items]
Other income, net	0	(200)
NSTAR Electric
Summary of Accounting Policies [Line Items]
Other income, net	12,238	11,086
NSTAR Electric | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other income, net	7,900	7,000
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	5,000	4,000
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	100	200
NSTAR Electric | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	(1,300)	(300)
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	100	200
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other income, net	400	0
PSNH
Summary of Accounting Policies [Line Items]
Other income, net	3,190	7,022
PSNH | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other income, net	1,900	500
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	1,500	200
PSNH | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	0	0
PSNH | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	(500)	400
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	300	5,900
PSNH | Other
Summary of Accounting Policies [Line Items]
Other income, net	$ 0	$ 0