SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Basis of Presentation (Details) customer in Millions, $ in Millions		9 Months Ended
	Jun. 02, 2017 USD ($)	Sep. 30, 2017 customer utility
Schedule of Equity Method Investments [Line Items]
Number of electric and natural gas customers | customer		3.7
Connecticut, Massachusetts and New Hampshire
Schedule of Equity Method Investments [Line Items]
Number of regulated utilities | utility		6
Aquarion Water Company
Schedule of Equity Method Investments [Line Items]
Amount involved in acquisition of Aquarion	$ 1,675
Cash paid to acquire Aquarion	880
Liability assumed in acquisition of Aquarion	$ 795