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Asset Retirement Obligations - Narrative (Details)
CAD in Millions
12 Months Ended
Dec. 31, 2015
CAD
CAD / mi
Dec. 31, 2014
CAD
Dec. 31, 2013
CAD
Asset Retirement Obligation [Line Items]      
Asset retirement obligations discounted rate 6.25%    
Fee payable per mile of abandoned track, levied under the Canada Transportation Act | CAD / mi 30,000    
Asset retirement obligations CAD 22 CAD 23 CAD 24
Grain-dependent branch lines Retirement Obligation Payments Time Period 2016 – 2044    
Joint facility retirement obligation payments time period 30 years    
Undiscounted [Member] | Grain-dependent branch lines [Member]      
Asset Retirement Obligation [Line Items]      
Asset retirement obligations CAD 38 38  
Undiscounted [Member] | Joint Facility [Member]      
Asset Retirement Obligation [Line Items]      
Asset retirement obligations 21 21  
Discounted [Member] | Grain-dependent branch lines [Member]      
Asset Retirement Obligation [Line Items]      
Asset retirement obligations 19 20  
Discounted [Member] | Joint Facility [Member]      
Asset Retirement Obligation [Line Items]      
Asset retirement obligations CAD 3 CAD 3