|**Basis of Presentation (Tables)**                                                  | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                            
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|**Organization, Consolidation and Presentation of Financial Statements [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Schedule of Cash and Cash Equivalents                                               | The following table reconciles cash and cash equivalents and restricted cash reported in our condensed consolidated balance sheets to the total amount presented in our condensed consolidated statements of cash flows:March 31,December 31,20182017amounts in thousandsCash and cash equivalents$402,672573,210Restricted cash included in other current assets970938Total cash and cash equivalents and restricted cash at end of period$403,642574,148
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|                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|                                                                                    | March 31,                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|                                                                                    | 2018                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|                                                                                    | amounts in thousands                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Cash and cash equivalents                                                           | $                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Restricted cash included in other current assets                                    | 970                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Total cash and cash equivalents and restricted cash at end of period                | $                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Schedule of Restricted Cash                                                         | The following table reconciles cash and cash equivalents and restricted cash reported in our condensed consolidated balance sheets to the total amount presented in our condensed consolidated statements of cash flows:March 31,December 31,20182017amounts in thousandsCash and cash equivalents$402,672573,210Restricted cash included in other current assets970938Total cash and cash equivalents and restricted cash at end of period$403,642574,148
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|                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|                                                                                    | March 31,                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|                                                                                    | 2018                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|                                                                                    | amounts in thousands                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Cash and cash equivalents                                                           | $                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Restricted cash included in other current assets                                    | 970                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Total cash and cash equivalents and restricted cash at end of period                | $                                                                                                                                                                                                                                                                                                                                                                                                                                                         

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