Stock-Based Compensation Stock-Based Compensation (Schedule of Restricted Stock Awards and Stock Options) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Abstract]
Recognized stock compensation expense	$ 2.1	$ 4.4	$ 11.2	$ 12.7
Capitalized stock compensation cost for our oil and natural gas properties	0.7	0.9	2.6	2.7
Tax benefit on stock based compensation	$ 0.8	$ 1.7	$ 4.2	$ 4.9