SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Summary of Accounting Policies [Line Items]
Other income, net	$ 30,243	$ 45,866	$ 54,347	$ 76,850
Pension, SERP and PBOP Non-Service Income/(Expense) Components
Summary of Accounting Policies [Line Items]
Other income, net	10,700	5,800	23,400	13,100
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	10,800	13,100	21,400	24,100
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	5,900	25,900	9,800	30,900
Unrealized gain associated with investment	2,400	20,400	2,400	20,400
Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	1,800	(600)	(2,500)	600
Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	1,000	1,300	1,800	7,800
Other
Summary of Accounting Policies [Line Items]
Other income, net	0	400	400	400
CL&P
Summary of Accounting Policies [Line Items]
Other income, net	8,459	2,853	10,357	6,733
CL&P | Pension, SERP and PBOP Non-Service Income/(Expense) Components
Summary of Accounting Policies [Line Items]
Other income, net	900	(900)	2,100	(1,600)
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	3,900	3,200	7,900	5,800
CL&P | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	0	100	0	100
CL&P | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	2,200	(100)	(1,300)	1,700
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	1,400	500	1,600	800
CL&P | Other
Summary of Accounting Policies [Line Items]
Other income, net	100	100	100	(100)
NSTAR Electric
Summary of Accounting Policies [Line Items]
Other income, net	13,112	10,657	25,350	21,743
NSTAR Electric | Pension, SERP and PBOP Non-Service Income/(Expense) Components
Summary of Accounting Policies [Line Items]
Other income, net	7,100	5,400	15,000	12,400
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	5,300	5,100	10,300	9,100
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	100	200	200	400
NSTAR Electric | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	0	(300)	(1,300)	(600)
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	500	200	700	400
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other income, net	100	100	500	0
PSNH
Summary of Accounting Policies [Line Items]
Other income, net	3,626	2,984	6,817	10,006
PSNH | Pension, SERP and PBOP Non-Service Income/(Expense) Components
Summary of Accounting Policies [Line Items]
Other income, net	1,600	1,500	3,500	2,000
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	1,200	900	2,600	1,100
PSNH | Equity in Earnings of Unconsolidated Affiliates
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	(100)	(200)	200
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	500	700	800	6,600
PSNH | Other
Summary of Accounting Policies [Line Items]
Other income, net	$ 0	$ 0	$ 100	$ 100