Revenues - Disaggregation of Revenue (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 1,921	$ 1,848	$ 5,897	$ 5,653
Leasing revenues	21	15	58	45
Total revenues	1,942	1,863	5,955	5,698
Freight
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,896	1,821	5,822	5,573
Total revenues	1,896	1,821	5,822	5,573
Grain
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	352	457	1,244	1,321
Coal
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	158	130	491	411
Potash
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	113	132	348	390
Fertilizers and sulphur
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	72	65	227	212
Forest products
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	89	85	259	244
Energy, chemicals and plastics
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	392	321	1,149	1,153
Metals, minerals and consumer products
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	196	152	535	474
Automotive
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	83	94	289	215
Intermodal
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	441	385	1,280	1,153
Non-freight excluding leasing revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 25	$ 27	$ 75	$ 80