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Segment Information (Revenue by Customer Type) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Revenue, Major Customer [Line Items]    
Revenue $ 719,143 $ 697,192
Strategic Service Provider [Member]    
Revenue, Major Customer [Line Items]    
Revenue 22,101 62,316
Non-strategic Service Provider [Member]    
Revenue, Major Customer [Line Items]    
Revenue $ 697,042 $ 634,876