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SIGNIFICANT ACCOUNTING POLICIES - Revenue Recognition (Details) - Portland Natural Gas Transmission System
$ in Millions
 1 Months Ended 12 Months Ended
Feb. 19, 2015
USD ($)
Jan. 31, 2015
item
Dec. 31, 2014
item
Revenue Recognition      
Outstanding rate cases | item   2 2
Maximum number of days to refund to customers 60 days    
Accumulated refunds $ 114.3    
Interest on refund $ 8.0