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Summary of Restricted Stock Unit Activity (Detail) - $ / shares
shares in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
RSUs shares      
Nonvested at beginning of year 1.9 2.4 2.2
Granted 0.6 0.7 0.9
Vested (0.7) (0.8) (0.5)
Forfeited (0.2) (0.4) (0.2)
Nonvested at end of year 1.6 1.9 2.4
RSUs weighted-average grant date fair value      
Nonvested at beginning of year $ 94.13 $ 83.50 $ 83.49
Granted 123.98 109.86 87.03
Vested 87.18 79.54 88.78
Forfeited 117.24 88.45 85.82
Nonvested at end of year $ 107.34 $ 94.13 $ 83.50