Revenues - Disaggregation of Revenue (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 1,942	$ 2,029
Leasing revenues	17	14
Total revenues	1,959	2,043
Freight
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,918	2,000
Total revenues	1,918	2,000
Grain
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	448	418
Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	163	150
Potash
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	101	112
Fertilizers and sulphur
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	77	70
Forest products
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	80	78
Energy, chemicals and plastics
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	388	491
Metals, minerals and consumer products
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	159	189
Automotive
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	108	87
Intermodal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	394	405
Non-freight excluding leasing revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 24	$ 29