SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Summary of Accounting Policies [Line Items]
Other income, net	$ 45,866	$ 50,149	$ 76,850	$ 83,938
Pension, SERP and PBOP Non-Service Income/(Expense) Components
Summary of Accounting Policies [Line Items]
Other income, net	5,800	14,600	13,100	29,800
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	13,100	10,900	24,100	20,600
Equity in Earnings
Summary of Accounting Policies [Line Items]
Other income, net	25,900	22,900	30,900	27,500
Unrealized gain associated with investment	20,400	17,600	20,400	17,600
Investment Income
Summary of Accounting Policies [Line Items]
Other income, net	(600)	(300)	600	400
Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	1,300	1,900	7,800	5,400
Other
Summary of Accounting Policies [Line Items]
Other income, net	400	100	400	200
CL&P
Summary of Accounting Policies [Line Items]
Other income, net	2,853	7,063	6,733	13,623
CL&P | Pension, SERP and PBOP Non-Service Income/(Expense) Components
Summary of Accounting Policies [Line Items]
Other income, net	(900)	2,400	(1,600)	5,300
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	3,200	3,300	5,800	6,100
CL&P | Equity in Earnings
Summary of Accounting Policies [Line Items]
Other income, net	100	0	100	0
CL&P | Investment Income
Summary of Accounting Policies [Line Items]
Other income, net	(100)	400	1,700	200
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	500	1,000	800	2,000
CL&P | Other
Summary of Accounting Policies [Line Items]
Other income, net	100	0	(100)	0
NSTAR Electric
Summary of Accounting Policies [Line Items]
Other income, net	10,657	14,269	21,743	26,870
NSTAR Electric | Pension, SERP and PBOP Non-Service Income/(Expense) Components
Summary of Accounting Policies [Line Items]
Other income, net	5,400	9,500	12,400	17,900
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	5,100	3,900	9,100	7,300
NSTAR Electric | Equity in Earnings
Summary of Accounting Policies [Line Items]
Other income, net	200	400	400	400
NSTAR Electric | Investment Income
Summary of Accounting Policies [Line Items]
Other income, net	(300)	300	(600)	900
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	200	200	400	400
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other income, net	100	0	0	0
PSNH
Summary of Accounting Policies [Line Items]
Other income, net	2,984	3,409	10,006	8,159
PSNH | Pension, SERP and PBOP Non-Service Income/(Expense) Components
Summary of Accounting Policies [Line Items]
Other income, net	1,500	2,100	2,000	4,400
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	900	0	1,100	0
PSNH | Equity in Earnings
Summary of Accounting Policies [Line Items]
Other income, net	0	0	0	0
PSNH | Investment Income
Summary of Accounting Policies [Line Items]
Other income, net	(100)	0	200	100
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	700	1,300	6,600	3,700
PSNH | Other
Summary of Accounting Policies [Line Items]
Other income, net	$ 0	$ 0	$ 100	$ 0