Properties - Net Properties (Details) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Cost	CAD 24,429	CAD 23,814
Accumulated depreciation	7,413	7,125
Net book value	17,016	16,689
Depreciation expense	CAD 661	640	CAD 595
Track and railway [Member]
Property, Plant and Equipment [Line Items]
Average annual depreciation rate	2.80%
Cost	CAD 17,285	16,817
Accumulated depreciation	4,814	4,573
Net book value	CAD 12,471	12,244
Buildings [Member]
Property, Plant and Equipment [Line Items]
Average annual depreciation rate	3.00%
Cost	CAD 719	662
Accumulated depreciation	196	178
Net book value	CAD 523	484
Rolling stock [Member]
Property, Plant and Equipment [Line Items]
Average annual depreciation rate	2.90%
Cost	CAD 4,114	4,060
Accumulated depreciation	1,557	1,524
Net book value	CAD 2,557	2,536
Information systems [Member]
Property, Plant and Equipment [Line Items]
Average annual depreciation rate	11.40%
Cost	CAD 551	584
Accumulated depreciation	264	299
Net book value	CAD 287	285
Other [Member]
Property, Plant and Equipment [Line Items]
Average annual depreciation rate	5.10%
Cost	CAD 1,760	1,691
Accumulated depreciation	582	551
Net book value	1,178	1,140
Software Development [Member]
Property, Plant and Equipment [Line Items]
Capitalized costs attributable to design and development	49	46	42
Depreciation expense	CAD 55	CAD 63	CAD 69