Annual report pursuant to Section 13 and 15(d)

Stockholders' Equity - Narrative (Details)

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Stockholders' Equity - Narrative (Details) - USD ($)
1 Months Ended 12 Months Ended
May 31, 2012
May 31, 2016
Sep. 30, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Corporate Share Repurchase            
Shares repurchased in period (shares)       14,000 40,500 6,120
Shares repurchased in period, value       $ 1,966,000 $ 3,901,000 $ 489,000
Stock repurchase program, authorized amount     $ 200,000,000.0      
Stock repurchase program, period in force     3 years      
Stock repurchase program, remaining authorized repurchase amount       $ 193,600,000    
Stock Options            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Weighted-average grant date fair value per share of stock options (in USD per share)       $ 53.17 $ 46.53 $ 35.39
Intrinsic value of options exercised       $ 13,300,000 $ 12,000,000 $ 20,700,000
Cash received from options exercised       4,700,000 $ 6,200,000 $ 8,700,000
Unrecognized compensation cost       $ 19,400,000    
Weighted-average period in which cost is expected to be recognized       2 years 5 months 37 days    
Restricted Stock            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Weighted-average period in which cost is expected to be recognized       1 year 3 months 33 days    
Unrecognized compensation cost, restricted stock       $ 6,500,000    
2002 Stock Incentive Plan | Stock Options            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Increase the number of shares under the 2002 Stock Incentive Plan (shares) 1,800,000 900,000        
Shares available for future option grants (shares)       800,000    
Amended ESPP            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares available for future option grants (shares)       67,394    
Employee Stock Purchase Plan            
Shares allowed to purchase in employee stock purchase plan per employee (shares)       1,250    
Common stock issued under amended ESSP (shares)       3,061 1,961 3,374