Shareholders' Equity - Activities Under Shares Repurchase Program (Detail) - CAD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of Common Shares repurchased (in shares)	4,600,000	1,900,000	6,900,000
CP Common Shares repurchased	$ 1,127	$ 381	$ 1,210
2017 and Current Normal Course Issuer Bid (NCIB) [Member] [Domain]
Number of Common Shares repurchased (in shares)	4,683,162
Weighted-average price per share	$ 240.68
CP Common Shares repurchased	$ 1,127
2017 Normal Course Issuer Bid (NCIB) [Member]
Number of Common Shares repurchased (in shares)		1,888,100
Weighted-average price per share		$ 201.53
CP Common Shares repurchased		$ 381
2016 Normal Course Issuer Bid (NCIB) [Member]
Number of Common Shares repurchased (in shares)			6,910,000
Weighted-average price per share			$ 175.08
CP Common Shares repurchased			$ 1,210