SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands, customer in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) customer utility MW	Jun. 30, 2017 USD ($)	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	$ 3,000		$ 22,000
Unrealized losses recorded in other income	900		200
Increase in regulatory liabilities			20,000
Other Income, Net	50,149	$ 29,022	$ 83,938	$ 50,641
Capacity required for transmission line | MW			1,090
Amount capitalized in transmission line installation			$ 297,000
Accounting Standards Update 2016-15
Schedule of Equity Method Investments [Line Items]
Dividends from equity method investment			10,400	9,400
New accounting guidance for statement of cash flows
Schedule of Equity Method Investments [Line Items]
Restricted cash		(22,400)		(22,400)
The Connecticut Light and Power Company
Schedule of Equity Method Investments [Line Items]
Other Income, Net	7,063	4,221	13,623	7,489
NSTAR Electric Company
Schedule of Equity Method Investments [Line Items]
Other Income, Net	14,269	7,096	26,870	15,445
Public Service Company of New Hampshire
Schedule of Equity Method Investments [Line Items]
Other Income, Net	3,409	1,891	8,159	4,703
Non-Service Cost Benefits
Schedule of Equity Method Investments [Line Items]
Other Income, Net	14,600	7,500	29,800	15,500
Non-Service Cost Benefits | The Connecticut Light and Power Company
Schedule of Equity Method Investments [Line Items]
Other Income, Net	2,400	400	5,300	900
Non-Service Cost Benefits | NSTAR Electric Company
Schedule of Equity Method Investments [Line Items]
Other Income, Net	9,500	4,800	17,900	9,800
Non-Service Cost Benefits | Public Service Company of New Hampshire
Schedule of Equity Method Investments [Line Items]
Other Income, Net	$ 2,100	$ 1,500	$ 4,400	$ 3,100
Connecticut, Massachusetts and New Hampshire
Schedule of Equity Method Investments [Line Items]
Number of regulated utilities | utility			8