Revenues - Disaggregation of Revenue (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 2,034	$ 1,777	$ 3,976	$ 3,806
Leasing revenues	20	15	37	29
Total revenues	2,054	1,792	4,013	3,835
Freight
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,008	1,752	3,926	3,752
Total revenues	2,008	1,752	3,926	3,752
Grain
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	444	446	892	864
Coal
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	170	131	333	281
Potash
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	134	146	235	258
Fertilizers and sulphur
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	78	77	155	147
Forest products
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	90	81	170	159
Energy, chemicals and plastics
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	369	341	757	832
Metals, minerals and consumer products
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	180	133	339	322
Automotive
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	98	34	206	121
Intermodal
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	445	363	839	768
Non-freight excluding leasing revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 26	$ 25	$ 50	$ 54