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Stock-Based Compensation Stock-Based Compensation (Schedule of Restricted Stock Awards and Stock Options) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Abstract]        
Recognized stock compensation expense $ 4.8 $ 4.5 $ 9.1 $ 8.3
Capitalized stock compensation cost for our oil and natural gas properties 1.0 1.0 1.9 1.8
Tax benefit on stock based compensation $ 1.7 $ 1.7 $ 3.4 $ 3.2