Shareholders' Equity - Activities Under Shares Repurchase Program (Detail) - CAD CAD / shares in Units, CAD in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Common Shares repurchased (in shares)	1,900,000	6,900,000	13,700,000
CP Common Shares repurchased	CAD 381	CAD 1,210	CAD 2,748
Current Normal Course Issuer Bid (NCIB) [Member]
Number of Common Shares repurchased (in shares)	1,888,100
Treasury Stock Acquired, Average Cost Per Share	CAD 201.53
CP Common Shares repurchased	CAD 381
2016 Normal Course Issuer Bid (NCIB) [Member] [Domain]
Number of Common Shares repurchased (in shares)		6,910,000
Treasury Stock Acquired, Average Cost Per Share		CAD 175.08
CP Common Shares repurchased		CAD 1,210
Amended and Normal Course Issuer Bid (NCIB) [Member] [Member]
Number of Common Shares repurchased (in shares)			13,549,977
Treasury Stock Acquired, Average Cost Per Share			CAD 202.79
CP Common Shares repurchased			CAD 2,748