Summary of Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2018 mi
Summary of Significant Accounting Policies [Line items]
Miles of railway	12,500
Short-term investments maturity, maximum	3 months
Expected return on fund assets, Market-related asset values used	five-year average of market values
Market-related asset value, Maximum percentage	120.00%
Market-related asset value, Minimum percentage	80.00%
Unrecognized actuarial gains and losses, Maximum percentage	10.00%
Expected average remaining service period of active employees expected to receive benefits	12 years
Probability threshold for recognizing income tax benefits to be realized upon settlement	50.00%
Favourable leases, customer relationships and interline contracts [Member] | Minimum [Member]
Summary of Significant Accounting Policies [Line items]
Amortization period of intangible assets with finite lives (years)	15 years
Favourable leases, customer relationships and interline contracts [Member] | Maximum [Member]
Summary of Significant Accounting Policies [Line items]
Amortization period of intangible assets with finite lives (years)	20 years