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Revenue (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2018
Jan. 01, 2018
Capitalized contract costs $ 16,300 $ 16,300 $ 16,296
Capitalized contract costs amortization $ 4,666 $ 13,260  
Maximum [Member]      
Standard customer service agreement period   3 years  
Estimated life of relevant customer relationship   5 years  
Minimum [Member]      
Standard customer service agreement period   1 year  
Estimated life of relevant customer relationship   1 year