Properties - Net Properties (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Cost	$ 29,051	$ 27,255
Accumulated depreciation	8,629	8,099
Net book value	20,422	19,156
Capitalized internal-use software, Costs	45	55	$ 53
Capitalized internal-use software, Depreciation	$ 42	44	$ 49
Track and roadway
Property, Plant and Equipment [Line Items]
Weighted average annual depreciation rate	2.80%
Cost	$ 20,676	19,299
Accumulated depreciation	5,859	5,522
Net book value	$ 14,817	13,777
Buildings
Property, Plant and Equipment [Line Items]
Weighted average annual depreciation rate	2.90%
Cost	$ 937	833
Accumulated depreciation	259	237
Net book value	$ 678	596
Rolling stock
Property, Plant and Equipment [Line Items]
Weighted average annual depreciation rate	2.80%
Cost	$ 4,702	4,529
Accumulated depreciation	1,498	1,445
Net book value	$ 3,204	3,084
Information systems software
Property, Plant and Equipment [Line Items]
Weighted average annual depreciation rate	9.30%
Cost	$ 569	527
Accumulated depreciation	253	215
Net book value	$ 316	312
Other
Property, Plant and Equipment [Line Items]
Weighted average annual depreciation rate	5.20%
Cost	$ 2,167	2,067
Accumulated depreciation	760	680
Net book value	$ 1,407	$ 1,387