Equity impact of the value of stock that has been repurchased during the period and has not been retired and is not held in treasury. Some state laws may mandate the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

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-Name Accounting Standards Codification
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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Shareholders' equity - Activities Under Shares Repurchase Program (Details) - CAD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 19, 2020	Dec. 31, 2019
Number of Common Shares repurchased	0	20,400,000	21,400,000	19,000,000.0
Amount of repurchase		$ 1,477	$ 1,577	$ 1,140
2021 Normal Course Issuer Bid
Number of Common Shares repurchased	0
Weighted-average price per share	$ 0
Amount of repurchase	$ 0
2019 Normal Course Issuer Bid
Number of Common Shares repurchased		19,865,380
Weighted-average price per share		$ 74.35
Amount of repurchase		$ 1,477
2018 & 2019 Normal Course Issuer Bids
Number of Common Shares repurchased				18,970,745
Weighted-average price per share				$ 60.13
Amount of repurchase				$ 1,141