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Earnings Per Share - Computation of Earnings Per Share (Detail) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Basic earnings per Common Share:    
Net income attributable to Magna International Inc. $ 757 $ 1,765
Weighted average number of Common Shares outstanding during the year 299.7 314.7
Basic earnings per Common Share $ 2.52 $ 5.61
Diluted earnings per Common Share:    
Net income attributable to Magna International Inc. $ 757 $ 1,765
Weighted average number of Common Shares outstanding during the year 299.7 314.7
Stock options and restricted stock 0.7 1.1
Diluted 300.4 315.8
Diluted earnings per Common Share $ 2.52 $ 5.59