SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands, customer in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) customer utility MW	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Equity Method Investments [Line Items]
Number of electric and natural gas customers | customer			4
Undiscounted minimum rental payments, excluding executory costs, under long-term noncancelable operating and capital leases					$ 100,000
Increase in operating revenues and purchased power, fuel and transmission	$ 400		$ 22,400
Unrealized losses recorded in other income	2,400		2,600
Increase in regulatory liabilities			30,000
Other Income, Net	16,718	$ 28,536	$ 100,656	$ 79,178
Capacity required for transmission line | MW			1,090
Amount capitalized in transmission line installation			$ 302,000
Other than temporary impairment	32,900
Expenses recorded as taxes other than income taxes	10,700		36,100
Change in enacted tax rate	18,000
Accounting Standards Update 2016-15
Schedule of Equity Method Investments [Line Items]
Dividends from equity method investment			16,400	14,000
New accounting guidance for statement of cash flows
Schedule of Equity Method Investments [Line Items]
Restricted cash		(30,200)		(30,200)
The Connecticut Light and Power Company
Schedule of Equity Method Investments [Line Items]
Other Income, Net	7,098	7,913	20,722	15,402
NSTAR Electric Company
Schedule of Equity Method Investments [Line Items]
Other Income, Net	13,697	9,165	40,567	24,610
Public Service Company of New Hampshire
Schedule of Equity Method Investments [Line Items]
Other Income, Net	16,095	2,664	24,253	7,367
Non-Service Cost Benefits
Schedule of Equity Method Investments [Line Items]
Other Income, Net	14,800	7,300	44,600	22,900
Non-Service Cost Benefits | The Connecticut Light and Power Company
Schedule of Equity Method Investments [Line Items]
Other Income, Net	2,000	400	7,300	1,300
Non-Service Cost Benefits | NSTAR Electric Company
Schedule of Equity Method Investments [Line Items]
Other Income, Net	9,000	4,700	26,800	14,500
Non-Service Cost Benefits | Public Service Company of New Hampshire
Schedule of Equity Method Investments [Line Items]
Other Income, Net	$ 2,000	$ 1,500	$ 6,400	$ 4,500
Connecticut, Massachusetts and New Hampshire
Schedule of Equity Method Investments [Line Items]
Number of regulated utilities | utility			8
Access Northeast
Schedule of Equity Method Investments [Line Items]
Ownership percentage by parent	40.00%		40.00%