Summary of Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2016 mi
Finite-Lived Intangible Assets [Line Items]
Number of miles of railway	12,400
Cash and cash equivalents maturity period, maximum	3 months
Expected return on fund assets calculated using market-related asset values developed	five-year average of market values
The maximum market related asset value as a percentage of market value used to calculate net periodic benefit cost	120.00%
The minimum market related asset value as a percentage of market value used to calculate net periodic benefit cost	80.00%
Unrecognized actuarial gains and losses as a percentage in excess of the greater of the benefit obligation and the market-related plan assets	10.00%
Expected average remaining service period of active employees expected to receive benefits under the plan	11 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]
Finite-Lived Intangible Assets [Line Items]
Amortization period of intangible assets with finite lives (in years)	15 years
Favourable leases, customer relationships and interline contracts [Member] | Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization period of intangible assets with finite lives (in years)	20 years