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Stock-Based Compensation (Tables)
12 Months Ended
Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Stock-Based Compensation
Stock-based compensation is included in the following financial statement captions: 
 Year Ended December 31,
 201820172016
Cost of sales$6,535 $5,863 $6,018 
Sales and marketing2,550 2,041 1,820 
Research and development6,410 5,001 4,905 
General and administrative12,532 10,116 8,991 
Total stock-based compensation28,027 23,021 21,734 
Tax benefit recognized(6,632)(7,367)(6,971)
Net stock-based compensation$21,395 $15,654 $14,763 
Summary of Stock Option Valuation Assumptions The assumptions used in the Black-Scholes model or the calculation of compensation were as follows for the years ended December 31. 
201820172016
Expected term4.1 - 4.9 years3.8 - 5.0 years4.4 - 6.1 years
Volatility31% - 36%31% - 35%37% - 45%
Risk-free rate of return2.54% - 3.01%1.57% - 1.97%1.06% - 1.41%
Dividend yield0.25%  0.25%  0.25%  
Forfeiture rate—%  —%  2.65% - 5.26%
Summary of Option Activity
A summary of option activity is presented below (see Note 13, "Employee Benefit Plans" for further information):
Number of
Options
Weighted-
Average
Exercise
Price
Weighted-
Average
Remaining
Contractual
Life
Aggregate
Intrinsic
Value
   (In years)(In thousands)
Outstanding — January 1, 20162,224,169 $53.82 
Granted260,930 82.84 
Exercised(392,887)35.81 
Forfeited(27,959)72.87 
Outstanding — December 31, 20162,064,253 60.65 6.04$78,556 
Granted293,284 124.57 
Exercised(546,931)50.50 
Forfeited(13,113)90.81 
Outstanding — December 31, 20171,797,493 73.95 6.02$251,970 
Granted257,111 232.26 
Exercised(282,720)58.94 
Forfeited(24,810)131.36 
Outstanding — December 31, 20181,747,074 $98.93 5.80$58,084 
Unvested — December 31, 2018830,076 $140.80 7.82$9,179 
Exercisable — December 31, 2018916,998 $61.03 3.96$48,905 
Summary of Restricted Stock Unit Activity
The following table summarizes the restricted stock units ("RSU's") activity for the year ended December 31:
Number of
Shares
Weighted-Average Grant-Date Fair Value
Weighted-
Average
Remaining
Contractual
Life
Aggregate
Intrinsic
Value
   (In years)(In thousands)
Outstanding — January 1, 2016277,719 $77.22 
Granted140,452 81.86 
Converted(44,656)70.64 
Canceled(6,745)81.89 
Outstanding — December 31, 2016366,770 79.72 2.55$36,204 
Granted106,764 127.29 
Converted(90,385)66.18 
Canceled(4,888)90.54 
Outstanding — December 31, 2017378,261 96.23 2.55$80,997 
Granted80,254 227.45 
Converted(97,997)91.62 
Canceled(9,497)121.37 
Outstanding — December 31, 2018351,021 $126.93 2.62$39,767 
Unvested — December 31, 2018351,021 $126.93 2.62$39,767 
Summary of Performance Units Fair Value Assessment The weighted-average fair value of these performance units was determined using the Monte Carlo simulation model incorporating the following weighted-average assumptions:
201820172016
Performance term3.0 years3.0 years3.0 years
Volatility13% - 32%13% - 31%13% - 32%
Risk-free rate of return2.41%  1.49%  .88%  
Dividend yield—%  —%  —%  
Weighted-average fair value per share284.78 147.25 88.51 
Summary of Performance Stock Unit Activity
The following table summarizes the performance stock units ("PSU's") activity for the year ended December 31:
Number of
Shares
Weighted-Average Grant-Date Fair Value
Weighted-
Average
Remaining
Contractual
Life
Aggregate
Intrinsic
Value
   (In years)(In thousands)
Outstanding — January 1, 201627,233 $128.54 
Granted27,272 88.51 
Converted— 
Canceled— 
Outstanding — December 31, 201654,505 108.51 2.64$5,380 
Granted21,444 147.25 
Converted— 
Canceled— 
Outstanding — December 31, 201775,949 119.45 1.93$16,263 
Granted33,706 238.12 
Converted— 
Canceled— 
Outstanding — December 31, 2018109,655 $146.96 1.77$12,423 
Unvested — December 31, 2018109,655 $146.96 1.77$12,423