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ACQUISITIONS AND DISPOSITIONS - Assets Held for Sale (Narrative) (Details)
CAD in Millions, $ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2018
CAD
Mar. 31, 2018
USD ($)
Dec. 31, 2017
CAD
Dec. 31, 2016
CAD
Dec. 31, 2015
CAD
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Loss from remeasurement     CAD 4,463 CAD 1,376 CAD 96
Goodwill impairment     102 0 CAD 440
Gas Transmission and Midstream          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Loss from remeasurement     4,400    
After-tax loss from remeasurement     2,800    
Goodwill impairment     CAD 102 CAD 0  
Scenario, Forecast | Gas Distribution | Disposal group, held-for-sale, not discontinued operations | St. Lawrence Gas          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Cash proceeds from sale CAD 88 $ 70