SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 30,985	$ 33,789
Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	7,400	15,200
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	10,900	9,700
Equity in Earnings
Summary of Accounting Policies [Line Items]
Other Income, Net	5,000	4,600
Investment Income
Summary of Accounting Policies [Line Items]
Other Income, Net	1,200	700
Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	6,500	3,500
Other
Summary of Accounting Policies [Line Items]
Other Income, Net	0	100
CL&P
Summary of Accounting Policies [Line Items]
Other Income, Net	3,880	6,560
CL&P | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	(600)	3,000
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	2,600	2,800
CL&P | Equity in Earnings
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0
CL&P | Investment Income
Summary of Accounting Policies [Line Items]
Other Income, Net	1,700	(100)
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	400	900
CL&P | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	(200)	0
NSTAR Electric
Summary of Accounting Policies [Line Items]
Other Income, Net	11,086	12,601
NSTAR Electric | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	7,000	8,400
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	4,000	3,400
NSTAR Electric | Equity in Earnings
Summary of Accounting Policies [Line Items]
Other Income, Net	200	0
NSTAR Electric | Investment Income
Summary of Accounting Policies [Line Items]
Other Income, Net	(300)	600
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	200	200
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0
PSNH
Summary of Accounting Policies [Line Items]
Other Income, Net	7,022	4,749
PSNH | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	500	2,300
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	200	0
PSNH | Equity in Earnings
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0
PSNH | Investment Income
Summary of Accounting Policies [Line Items]
Other Income, Net	400	0
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	5,900	2,400
PSNH | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 0	$ 0