|**Long-Term Debt (Tables)**                              | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|-------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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|**Debt Disclosure [Abstract]**                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Schedule of Long-term Debt Instruments                   | Long-term debt consists of the following:September 30, 2013December 31, 2012(in thousands)Senior notes:3.25% Convertible senior notes due 2016:Principal amount$115,000$115,000Unamortized discount(10,950)(13,671)3.25% Convertible senior notes due 2016, net of discount104,050101,3297.75% Senior notes due 2022500,000500,000Total senior notes604,050601,329Credit facilities:Corporate—49,000PDCM34,75026,250Total credit facilities34,75075,250PDCM second lien term loan15,000—Total debt653,800676,579Less: Current portion of long-term debt104,050—Long-term debt$549,750$676,579
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|                                                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|                                                         | September 30, 2013                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|                                                         | (in thousands)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Senior notes:                                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|3.25% Convertible senior notes due 2016:                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Principal amount                                         | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Unamortized discount                                     | (10,950                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|3.25% Convertible senior notes due 2016, net of discount | 104,050                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|7.75% Senior notes due 2022                              | 500,000                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Total senior notes                                       | 604,050                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Credit facilities:                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Corporate                                                | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|PDCM                                                     | 34,750                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Total credit facilities                                  | 34,750                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|PDCM second lien term loan                               | 15,000                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Total debt                                               | 653,800                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Less: Current portion of long-term debt                  | 104,050                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Long-term debt                                           | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            

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