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CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical)
Dec. 31, 2017
Aug. 01, 2017
Jun. 01, 2017
Jan. 01, 2016
Interest acquired (as a percent)   49.34%    
Portland Natural Gas Transmission System        
Interest acquired (as a percent)     11.81% 49.90%
Iroquois        
Interest acquired (as a percent) 49.34%   49.34%