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Stock-Based Compensation - Summary of Restricted Stock Unit Activity (Detail) - Restricted Stock Units - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Restricted stock units outstanding, beginning balance (in shares) 1,347,879 1,416,438 1,403,974
Restricted stock units granted (in shares) 658,196 720,601 711,990
Restricted stock units, additional shares granted due to special distribution (in shares)   37 51,432
Restricted stock units released, vested (in shares) (606,064) (655,584) (623,554)
Special distribution shares released (in shares) (15,667) (35,354) (19,966)
Restricted stock units canceled (in shares) (79,451) (93,940) (103,922)
Special distribution shares canceled (in shares) (1,002) (4,319) (3,516)
Restricted stock units outstanding, ending balance (in shares) 1,303,891 1,347,879 1,416,438
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]      
Restricted stock units outstanding, weighted-average grant date fair value, beginning balance (in dollars per share) $ 192.59 $ 148.53 $ 114.56
Restricted stock units granted, weighted-average grant date fair value (in dollars per share) 389.60 309.18 236.89
Restricted stock units, additional stock units granted due to special distribution, weighted-average grant date fair value (in dollars per share)   297.03 297.03
Restricted stock units released, vested, weighted-average grant date fair value (in dollars per share) 260.75 213.72 173.79
Special distribution shares released, weighted-average grant date fair value (in dollars per share) 243.06 269.94 227.99
Restricted stock units canceled, weighted-average grant date fair value (in dollars per share) 313.83 242.41 198.67
Special distribution shares canceled, weighted-average grant date fair value (in dollars per share) 282.49 272.84 235.43
Restricted stock units outstanding, weighted-average grant date fair value, ending balance (in dollars per share) $ 252.30 $ 192.59 $ 148.53
Restricted stock units outstanding, weighted-average remaining contractual life (years) 1 year 2 months 19 days    
Restricted stock units outstanding, aggregate intrinsic value $ 590,950