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Commitments And Contingencies (Narrative) (Details)
number in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Commitments And Contingencies [Line Items]      
Number of domain names registered 131.9    
Uncertain tax positions $ 217,000,000    
ICANN Agreement [Member]      
Commitments And Contingencies [Line Items]      
Expiration Date of Registry Agreement Nov. 30, 2024    
Registry fee per transaction $ 0.25    
Payments for registry fees 32,300,000 $ 31,500,000 $ 30,900,000
.TV Agreement [Member]      
Commitments And Contingencies [Line Items]      
renewed agreement fees $ 5,000,000 $ 5,000,000 $ 5,000,000