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Stock-Based Compensation - Restricted Stock Units Activity (Detail) - Restricted Stock Units
12 Months Ended
Dec. 31, 2017
$ / shares
shares
Number of Restricted Stock Units  
Non-vested at December 31, 2016 | shares 723,398
Granted | shares 408,608
Vested | shares (338,988)
Forfeited | shares (64,953)
Non-vested at December 31, 2017 | shares 728,065
Weighted-Average Grant-Date Fair Value  
Non-vested at December 31, 2016 | $ / shares $ 49.22
Granted | $ / shares 58.59
Vested | $ / shares 48.13
Forfeited | $ / shares 53.56
Non-vested at December 31, 2017 | $ / shares $ 54.59