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SIGNIFICANT ACCOUNTING POLICIES - Basis of Presentation (Details)
Mar. 31, 2018
[1]
Jun. 01, 2017
Portland Natural Gas Transmission System    
ACQUISITIONS    
Interest acquired by Partnership (as a percent)   11.81%
Iroquois    
ACQUISITIONS    
Ownership interest (as a percent) 49.34% 49.34%
Portland Natural Gas Transmission System    
ACQUISITIONS    
Ownership interest (as a percent)   61.71%
[1] The Partnership acquired a 49.34% interest in Iroquois on June 1, 2017.