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Stock-Based Compensation (Narrative) (Details) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended 9 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Y
Sep. 30, 2011
Sep. 30, 2012
Minimum [Member]
Sep. 30, 2012
Maximum [Member]
May 02, 2012
Stock Options [Member]
Sep. 30, 2012
Performance Shares [Member]
First Half Of 2015 [Member]
Sep. 30, 2011
Performance Shares [Member]
First Half Of 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Compensation expense for restricted stock awards $ 2.9 $ 2.7 $ 8.2 $ 7.7          
Capitalized stock compensation cost for oil and natural gas properties 0.7 0.6 2.0 1.9          
Tax benefit on stock based compensation 1.2 1.1 3.2 3.0          
Unrecognized compensation cost related to unvested awards 14.5   14.5            
Unrecognized compensation cost, expected to be capitalized 2.6   2.6            
Weighted average years over which this cost will be recognized     0.8            
Number of shares authorized             3,300,000    
Restricted stock awards to be granted, percent               30.00% 30.00%
Shares granted               46,441 20,062
Performance evaluation period     3 years            
Performance percentage criteria         50.00% 150.00%      
Percentage of performance based shares received based on first year results           100.00%      
Increase in stock compensation expense due to the restricted stock awards granted     $ 5.6