Shareholders' Equity - Activities Under Shares Repurchase Program (Detail) - CAD CAD / shares in Units, CAD in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Number of Common Shares repurchased (in shares)		6,900,000	13,700,000	10,300,000
Current Normal Course Issuer Bid (NCIB) [Member]
Number of Common Shares repurchased (in shares)		6,910,000
Weighted-average price per share (in dollars per share)		CAD 175.08
Amount of repurchase		CAD 1,210
Amended and Normal Course Issuer Bid (NCIB) [Member] [Member]
Number of Common Shares repurchased (in shares)			13,549,977
Weighted-average price per share (in dollars per share)			CAD 202.79
Amount of repurchase			CAD 2,748
Normal Course Issuer Bid (NCIB) [Member]
Number of Common Shares repurchased (in shares)	2,200,000			10,476,074
Weighted-average price per share (in dollars per share)				CAD 199.42
Amount of repurchase	CAD 490			CAD 2,089