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Stockholders' Equity (Narrative) (Details) - USD ($)
1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended 36 Months Ended
Sep. 30, 2015
Dec. 31, 2015
Dec. 31, 2015
Dec. 31, 2015
Mar. 31, 2018
Jan. 30, 2015
Time-based equity award            
Stockholders' equity            
Compensation cost not yet recognized   $ 1,900,000 $ 1,900,000 $ 1,900,000    
Compensation expense     $ 1,300,000      
2015 Stock Incentive Plan            
Stockholders' equity            
Common stock reserved for issuance (in shares)   1,275,000 1,275,000 1,275,000    
Maximum number of shares that may be issued through the exercise of incentive stock options   750,000 750,000 750,000    
2015 Stock Incentive Plan | Time-based equity award            
Stockholders' equity            
Number of shares granted   18,000 30,000      
Weighted average grant date fair value (in USD per share)   $ 50.80 $ 56.06      
Award vesting period of the grants     3 years      
2015 Employee Stock Purchase Plan            
Stockholders' equity            
Common stock reserved for issuance (in shares) 1,000,000          
Employee stock purchase plan, purchase period 6 months          
Employee stock purchase plan discount 15.00%          
2006 Equity Incentive Plans | Nonvested stock units issued (NSUs)            
Stockholders' equity            
Number of shares granted     185,000      
Weighted average grant date fair value (in USD per share)     $ 74.22      
Percentage of revenue and EBITDA that must occur for vesting (at least)     33.33%      
2006 Equity Incentive Plans | Time-based equity award            
Stockholders' equity            
Number of shares granted     8,000      
Weighted average grant date fair value (in USD per share)     $ 73.43      
2016 Long-Term Incentive Awards            
Stockholders' equity            
Number of shares granted   308,000        
Weighted average grant date fair value (in USD per share)   $ 50.05        
Vesting rights, percentage   90.00%        
Compensation expense   $ 400,000 $ 2,200,000      
2015 Long-Term Incentive Awards            
Stockholders' equity            
Compensation expense     $ 400,000      
2015 Stock Repurchase Program            
Stockholders' equity            
Maximum stock repurchase amount approved by Board of Directors           $ 200,000,000
Treasury stock, shares, acquired     979,000      
Treasury stock, value, acquired     $ 69,201,000      
Treasury stock acquired, average cost per share (in USD per share)     $ 70.69 $ 71.64    
Treasury stock, number of shares held   1,356,000 1,356,000 1,356,000    
Treasury stock, value   $ 97,100,000 $ 97,100,000 $ 97,100,000    
Remaining stock repurchase amount approved by Board of Directors   $ 102,900,000 $ 102,900,000 $ 102,900,000    
Maximum | 2016 Long-Term Incentive Awards            
Stockholders' equity            
Percentage of RSUs vested above targeted amount if financial performance is achieved (up to)   200.00%        
Scenario, forecast [Member] | 2016 Long-Term Incentive Awards            
Stockholders' equity            
Performance period         36 months