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Stock-Based Compensation (Narrative) (Details) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
May 02, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Allocated share-based compensation expense $ 4.3 $ 3.0 $ 7.6 $ 5.3  
Capitalized stock compensation cost for oil and natural gas properties 0.9 0.7 1.6 1.3  
Tax benefit on stock based compensation 1.6 1.1 2.9 2.0  
Unrecognized compensation cost related to unvested awards 25.9   25.9    
Unrecognized compensation cost, expected to be capitalized 4.2   4.2    
Weighted average years over which this cost will be recognized     10 months 24 days    
Vesting period     3 years    
Increase in stock compensation expense due to the restricted stock awards granted     $ 4.0    
Minimum [Member]
         
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Performance percentage criteria     50.00%    
Percentage of performance based shares received based on most recent fiscal year results     100.00%    
Maximum [Member]
         
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Performance percentage criteria     150.00%    
Restricted Stock [Member]
         
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of shares authorized         3,300,000
Shares granted 21,128 24,606 469,677 392,542  
Performance Shares [Member]
         
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted stock awards granted in period, percent     30.00%    
Performance Shares [Member] | First Half Of 2016 [Member]
         
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Shares granted     57,405    
Performance Shares [Member] | First Half Of 2015 [Member]
         
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Shares granted     46,441