Debt - Components of Long-term Debt - B (Narrative) (Detail) - 5.41% Senior Secured Notes [Member] CAD in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2017 CAD	Dec. 31, 2016
Debt Instrument [Line Items]
Interest rate		5.41%	5.41%
Value of locomotive units used as collateral | CAD		CAD 118
Final repayment of principal amount | $	$ 44