SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Basis of Presentation (Details) customer in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2016 USD ($) customer utility
Schedule of Equity Method Investments [Line Items]
Number of electric and natural gas customers | customer	3.6
Connecticut, Massachusetts and New Hampshire [Member]
Schedule of Equity Method Investments [Line Items]
Number of regulated utilities | utility	6
Spectra Energy Corporation [Member] | Access Northeast [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest in Access Northeast	40.00%
Eversource [Member] | Access Northeast [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest in Access Northeast	40.00%
National Grid [Member] | Access Northeast [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest in Access Northeast	20.00%
Other Noncurrent Assets [Member] | Eversource [Member] | Access Northeast [Member]
Schedule of Equity Method Investments [Line Items]
Equity interest in project | $	$ 28.6