Quarterly report pursuant to Section 13 or 15(d)

Basis of Presentation (Cash and Cash Equivalents and Restricted Cash) (Details)

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Basis of Presentation (Cash and Cash Equivalents and Restricted Cash) (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]        
Cash and cash equivalents $ 689,562 $ 573,210    
Restricted cash included in other current assets 791 938    
Total cash and cash equivalents and restricted cash at end of period $ 690,353 $ 574,148 $ 512,873 $ 488,127