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Shareholders' Equity and Common Stock Equivalents - Narrative (Details) - USD ($)
12 Months Ended 72 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
Sep. 30, 2015
Dec. 31, 2014
Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Shares reacquired during period (in shares) 48,440 701,410 663,092 2,848,050      
Percentage of outstanding shares reacquired 0.10% 1.70% 1.60%        
Shares reacquired during period, value $ 1,660,000 $ 21,513,000 $ 21,950,000        
Share price of treasury stock acquired during period (in usd per share) $ 34.26 $ 30.65 $ 33.08 $ 25.33      
Remaining authorized repurchase amount $ 27,852,000     $ 27,852,000      
Treasury stock (in shares) 24,721,000 24,672,932 23,971,522 24,721,000   23,308,430  
Preferred stock, shares authorized (in shares) 1,000,000 1,000,000   1,000,000      
Preferred stock, par value (in usd per share) $ 0.001 $ 0.001   $ 0.001      
Intrinsic value 0            
Weighted average exercise prices of vested and exercisable options (in usd per share) 27.12 $ 22.73 $ 19.32 27.12      
Share price (in usd per share) $ 44.10     $ 44.10      
Aggregate intrinsic value of vested options $ 4,387,000     $ 4,387,000      
Aggregate intrinsic value of options $ 8,126,000     $ 8,126,000      
Minimum              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Expiration period 7 years            
Minimum | Employee Stock Option              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Vesting period 4 years            
Minimum | Restricted Stock Units              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Vesting period 1 year            
Maximum              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Expiration period 10 years            
Maximum | Restricted Stock Units              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Vesting period 5 years            
2011 Plan              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Authorized repurchase amount             $ 50,000,000
2015 Plan              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Authorized repurchase amount         $ 50,000,000    
Comprehensive Plan              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Amended increase in shares reserved for issuance (in shares) 3,250,000            
Shares available for grant (in shares) 2,391,000     2,391,000