Revenues - Disaggregation of Revenue (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
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									$ 7,916	$ 7,648	$ 7,729
Leasing revenues									79	62	63
Total revenues	$ 2,040	$ 1,942	$ 2,054	$ 1,959	$ 2,012	$ 1,863	$ 1,792	$ 2,043	7,995	7,710	7,792
Freight
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									7,816	7,541	7,613
Total revenues									7,816	7,541	7,613
Grain
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									1,684	1,829	1,684
Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									625	566	682
Potash
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									463	493	462
Fertilizers and sulphur
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									305	290	250
Forest products
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									348	328	304
Energy, chemicals and plastics
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									1,563	1,519	1,534
Metals, minerals and consumer products
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									728	629	752
Automotive
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									376	324	352
Intermodal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									1,724	1,563	1,593
Non-freight excluding leasing revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									$ 100	$ 107	$ 116