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Stock-Based Compensation (Narrative) (Details) - USD ($)
3 Months Ended 12 Months Ended
May 23, 2018
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
May 20, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Forfeiture period once emplyment has ended       90 days      
Weighted-average fair value of options granted       $ 33.01 $ 23.88 $ 19.19  
Total intrinsic value for options exercised       $ 78,000,000 $ 37,200,000 $ 36,800,000  
Cash received from option exercises       74,700,000 56,500,000 27,400,000  
Tax benefit realized from stock option exercises       $ 0 0 0  
Share price       $ 161.89      
Total fair value of shares vested       $ 24,000,000 21,400,000 18,500,000  
Non-cash compensation expense       42,327,000 38,249,000 32,915,000  
Non-cash compensation capitalized to fixed and intangible assets       800,000 600,000 500,000  
Scenario, Forecast [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Accelerated compensation cost   $ 7,300,000 $ 11,200,000        
Stock Options [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Total unrecognized compensation cost related to unvested stock options       $ 43,300,000      
Weighted average period to recognize cost       2 years 6 months      
Restricted Stock Units (RSUs) [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Weighted average period to recognize cost       2 years 7 months 6 days      
Total unrecognized compensation expense related to unvested restricted stock       $ 28,100,000      
2010 Plan [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Maximum issuance of granted       7,500,000      
Shares remaining available for future issuance under the plan       6,500,000      
2010 Plan [Member] | Class A Common Stock [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Maximum issuance of shares       15,000,000     15,000,000
Shares remaining available for future issuance under the plan       6,500,000      
2008 Plan [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Shares remaining available for future issuance under the plan       289,948      
Percentage of purchase plan price per share equal to the fair market value       85.00%      
Non-cash compensation expense       $ 600,000 $ 600,000 $ 500,000  
2008 Plan [Member] | Class A Common Stock [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Maximum issuance of shares       500,000      
Class A common stock issued under the purchase plan       16,798 28,232    
Cash proceeds from issuance of shares under the purchase plan       $ 2,300,000 $ 3,300,000    
2018 Plan [Member] | Class A Common Stock [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Shares remaining available for future issuance under the plan 300,000            
Percentage of purchase plan price per share equal to the fair market value 85.00%            
Class A common stock issued under the purchase plan       10,052      
Cash proceeds from issuance of shares under the purchase plan       $ 1,400,000      
Minimum [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Contractual life of options and restricted stock units       7 years      
Maximum [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Contractual life of options and restricted stock units       10 years