INTERIM CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (unaudited) - CAD ($) shares in Millions, $ in Millions	Total	Share capital	Additional paid-in capital	Accumulated other comprehensive loss	Retained earnings	Cumulative Effect, Period of Adoption, Adjusted Balance	Cumulative Effect, Period of Adoption, Adjusted Balance Share capital	Cumulative Effect, Period of Adoption, Adjusted Balance Additional paid-in capital	Cumulative Effect, Period of Adoption, Adjusted Balance Accumulated other comprehensive loss	Cumulative Effect, Period of Adoption, Adjusted Balance Retained earnings
Beginning balance (shares) at Dec. 31, 2018	140.5
Balance at January 1 at Dec. 31, 2018						$ 6,631	$ 2,002	$ 42	$ (2,043)	$ 6,630
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 1,776	$ 0	$ 0	$ 0	$ 1,776
Other comprehensive income	51	0	0	51	0
Dividends declared	(320)	0	0	0	(320)
Effect of stock-based compensation expense	$ 11	0	11	0	0
CP Common Shares repurchased (shares)	(3.2)
CP Common Shares repurchased	$ (952)	(46)	0	0	(906)
Shares issued under stock option plan (shares)	0.2
Shares issued under stock option plan	$ 18	26	(8)	0	0
Ending balance (shares) at Sep. 30, 2019	137.5
Ending balance at Sep. 30, 2019	$ 7,215	1,982	45	(1,992)	7,180
Beginning balance (shares) at Jun. 30, 2019	139.1
Beginning balance at Jun. 30, 2019	$ 7,157	1,996	45	(2,009)	7,125
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	618	0	0	0	618
Other comprehensive income	17	0	0	17	0
Dividends declared	(114)	0	0	0	(114)
Effect of stock-based compensation expense	$ 3	0	3	0	0
CP Common Shares repurchased (shares)	(1.6)
CP Common Shares repurchased	$ (471)	(22)	0	0	(449)
Shares issued under stock option plan (shares)	0.0
Shares issued under stock option plan	$ 5	8	(3)	0	0
Ending balance (shares) at Sep. 30, 2019	137.5
Ending balance at Sep. 30, 2019	$ 7,215	1,982	45	(1,992)	7,180
Beginning balance (shares) at Dec. 31, 2019	137.0
Beginning balance at Dec. 31, 2019	$ 7,069	1,993	48	(2,522)	7,550
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,642	0	0	0	1,642
Other comprehensive income	106	0	0	106	0
Dividends declared	(353)	0	0	0	(353)
Effect of stock-based compensation expense	$ 13	0	13	0	0
CP Common Shares repurchased (shares)	(2.7)
CP Common Shares repurchased	$ (916)	(39)	0	0	(877)
Shares issued under stock option plan (shares)	0.2
Shares issued under stock option plan	$ 19	24	(5)	0	0
Ending balance (shares) at Sep. 30, 2020	134.5
Ending balance at Sep. 30, 2020	$ 7,579	1,978	56	(2,416)	7,961
Beginning balance (shares) at Jun. 30, 2020	135.5
Beginning balance at Jun. 30, 2020	$ 7,465	1,990	53	(2,450)	7,872
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	598	0	0	0	598
Other comprehensive income	34	0	0	34	0
Dividends declared	(128)	0	0	0	(128)
Effect of stock-based compensation expense	$ 4	0	4	0	0
CP Common Shares repurchased (shares)	(1.0)
CP Common Shares repurchased	$ (396)	(15)	0	0	(381)
Shares issued under stock option plan (shares)	0.0
Shares issued under stock option plan	$ 2	3	(1)	0	0
Ending balance (shares) at Sep. 30, 2020	134.5
Ending balance at Sep. 30, 2020	$ 7,579	$ 1,978	$ 56	$ (2,416)	$ 7,961