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