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Summary of Restricted Stock Unit Activity (Detail) - $ / shares
shares in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
RSUs shares      
Nonvested at beginning of year 1.4 1.5 1.6
Granted 0.6 0.6 0.6
Vested (0.6) (0.6) (0.6)
Forfeited (0.1) (0.1) (0.1)
Nonvested at end of year 1.3 1.4 1.5
RSUs weighted-average grant date fair value      
Nonvested at beginning of year $ 150.95 $ 132.56 $ 107.34
Granted 201.92 171.48 158.28
Vested 127.99 116.42 91.20
Forfeited 172.45 153.58 132.14
Nonvested at end of year $ 176.81 $ 150.95 $ 132.56