- Disaggregation of Revenue (Details) - CAD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									$ 7,648	$ 7,729	$ 7,254
Leasing revenues									62	63	62
Total revenues	$ 2,012	$ 1,863	$ 1,792	$ 2,043	$ 2,069	$ 1,979	$ 1,977	$ 1,767	7,710	7,792	7,316
Freight
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									7,541	7,613	7,152
Total revenues									7,541	7,613	7,152
Grain
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									1,829	1,684	1,566
Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									566	682	673
Potash
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									493	462	486
Fertilizers and sulphur
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									290	250	243
Forest products
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									328	304	284
Energy, chemicals and plastics
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									1,519	1,534	1,243
Metals, minerals and consumer products
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									629	752	797
Automotive
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									324	352	322
Intermodal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									1,563	1,593	1,538
Non-freight excluding leasing revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers									$ 107	$ 116	$ 102