Basis of Presentation and Significant Accounting Policies (Details 4) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended		12 Months Ended
	Mar. 31, 2010	Jun. 30, 2011	Dec. 31, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Summary of Significant Accounting Policies (Details)
Hurricane related repair expenses						$ 8.2
Estimated future hurricane repair expenses				27.5
Insurance proceeds associated with Hurricane Ike damage					47.8
Amount of insurance proceeds collected that exceeds repair costs incurred	39.8
Pretax insurance gains credited to earnings					2.2
Hurricane damage repairs for which remaining insurance coverage is available				19.5
Insurance proceeds received under settlement agreement			40
Anticipated insurance recoveries reflected as receivables				1.1	0.5
Insurance gains credited to earnings					2.5	5.4
Fair value of Senior Notes				332.4	315.7
Carrying value of Senior Notes				305	305
Cash paid for liquidation of hedge contract		14.2
Impairments of oil and gas properties				$ 15.2	$ 63.8	$ 11.4
Aggregate percentage of sold reserves				95.00%