Segmented and geographic information - Geographic Information (Details) - CAD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	$ 2,040	$ 1,942	$ 2,054	$ 1,959	$ 2,012	$ 1,863	$ 1,792	$ 2,043	$ 7,995	$ 7,710	$ 7,792
Long-term assets excluding investment in Kansas City Southern, financial instruments, and pension assets	22,196				21,423				22,196	21,423	20,151
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									5,992	5,829	5,675
Long-term assets excluding investment in Kansas City Southern, financial instruments, and pension assets	14,922				14,258				14,922	14,258	13,131
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									2,003	1,881	2,117
Long-term assets excluding investment in Kansas City Southern, financial instruments, and pension assets	$ 7,274				$ 7,165				$ 7,274	$ 7,165	$ 7,020