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 2 Months Ended 12 Months Ended
May 31, 2012
May 31, 2018
May 31, 2016
Sep. 30, 2015
Feb. 28, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Jan. 23, 2019
Corporate Share Repurchase                  
Shares repurchased in period (shares)   260,000       782,248 14,000 40,500  
Price per share of shares repurchased (USD per share)   $ 191.14              
Shares repurchased in period           $ 127,481,000 $ 1,966,000 $ 3,901,000  
Stock repurchase program, authorized amount       $ 200,000,000.0          
Stock repurchase program, period in force       3 years          
Stock repurchase program, remaining authorized repurchase amount           $ 125,200,000      
Stock Options                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Weighted-average grant date fair value per share of stock options (USD per share)           $ 66.71 $ 53.17 $ 46.53  
Intrinsic value of options exercised           $ 51,900,000 $ 13,300,000 $ 12,000,000.0  
Cash received from options exercised           19,800,000 $ 4,700,000 $ 6,200,000  
Unrecognized compensation cost           $ 20,400,000      
Weighted-average period in which cost is expected to be recognized           2 years 4 months 20 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 4 months 17 days      
Unrecognized compensation cost, restricted stock           $ 9,200,000      
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)           600,000      
Amended ESPP                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Shares available for future option grants (shares)           64,008      
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)           3,386 3,061 1,961  
Subsequent Event                  
Corporate Share Repurchase                  
Shares repurchased in period (shares)         400,177        
Stock repurchase program, authorized amount         $ 225,900,000       $ 350,000,000.0