Shareholders' Equity - Narrative (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 19, 2020	Oct. 01, 2021	Jan. 29, 2021	Dec. 20, 2019
Common Shares repurchased		5,300,000		13,700,000
Common Shares repurchased (value)		$ 396		$ 916
KCS | Subsequent Event
Common shares to be issued to KCS						264,700,000
2021 Normal Course Issuer Bid (NCIB)
Common Shares authorized to be repurchased							16,700,000
Common Shares repurchased	0		0
Common Shares repurchased (value)	$ 0		$ 0
2019 Normal Course Issuer Bid (NCIB)
Common Shares authorized to be repurchased								24,000,000
Common Shares repurchased		5,224,340		13,257,910	21,400,000
Common Shares repurchased (value)		$ 396		$ 916	$ 1,577