SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Basis of Presentation (Details) $ in Thousands, customer in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) MW	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) customer utility
Schedule of Equity Method Investments [Line Items]
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	$ 19,000
Number of electric and natural gas customers | customer			4
Increase in regulatory liabilities	10,000
Other Income, Net	$ 33,789	$ 21,619
Capacity required for transmission line | MW	1,090
Amount capitalized in transmission line installation	$ 289,000
Connecticut, Massachusetts and New Hampshire
Schedule of Equity Method Investments [Line Items]
Number of regulated utilities | utility			8
Accounting Standards Update 2016-01
Schedule of Equity Method Investments [Line Items]
Unrealized losses recorded in other income	700
Accounting Standards Update 2016-15
Schedule of Equity Method Investments [Line Items]
New guidance on equity method investments	$ 6,200	$ 4,800