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Stock-Based Compensation (Narrative) (Details) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
May 02, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share-based compensation expense $ 3.8 $ 3.3  
Capitalized stock compensation cost for oil and natural gas properties 0.8 0.7  
Tax benefit on stock based compensation 1.5 1.3  
Unrecognized compensation cost related to unvested awards 31.8    
Unrecognized compensation cost, expected to be capitalized 5.4    
Weighted average years over which this cost will be recognized 1 year    
Increase in stock compensation expense due to the restricted stock awards granted $ 1.0    
Minimum [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Performance percentage criteria 0.00%    
Percentage of performance based shares received based on first 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 438,342 448,549  
Performance Shares [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted stock awards granted in period, percent 40.00% 30.00%  
Performance Shares [Member] | First Half Of 2017 [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares granted 71,674    
Performance Shares [Member] | First Half Of 2016 [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares granted   57,405