|**Long-Term Debt (Tables)**                              | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|-------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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|**Debt Disclosure [Abstract]**                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Schedule of Long-term Debt Instruments                   | Long-term debt consists of the following:June 30, 2013December 31, 2012(in thousands)Senior notes:3.25% Convertible senior notes due 2016:Principal amount$115,000$115,000Unamortized discount(11,873)(13,671)3.25% Convertible senior notes due 2016, net of discount103,127101,3297.75% Senior notes due 2022500,000500,000Total senior notes603,127601,329Credit facilities:Corporate—49,000PDCM36,00026,250Total credit facilities36,00075,250Total long-term debt$639,127$676,579
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|                                                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|                                                         | June 30, 2013                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|                                                         | (in thousands)                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Senior notes:                                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|3.25% Convertible senior notes due 2016:                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Principal amount                                         | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Unamortized discount                                     | (11,873                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|3.25% Convertible senior notes due 2016, net of discount | 103,127                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|7.75% Senior notes due 2022                              | 500,000                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Total senior notes                                       | 603,127                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Credit facilities:                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Corporate                                                | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|PDCM                                                     | 36,000                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Total credit facilities                                  | 36,000                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Total long-term debt                                     | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|                                                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       

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