Revenues - Disaggregation of Revenue (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 1,848	$ 1,964	$ 5,653	$ 5,678
Leasing revenues	15	15	45	45
Total revenues	1,863	1,979	5,698	5,723
Freight
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,821	1,932	5,573	5,589
Total revenues	1,821	1,932	5,573	5,589
Grain
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	457	409	1,321	1,211
Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	130	183	411	514
Potash
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	132	117	390	367
Fertilizers and sulphur
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	65	66	212	186
Forest products
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	85	78	244	229
Energy, chemicals and plastics
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	321	382	1,153	1,043
Metals, minerals and consumer products
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	152	201	474	579
Automotive
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	94	87	215	267
Intermodal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	385	409	1,153	1,193
Non-freight excluding leasing revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 27	$ 32	$ 80	$ 89