Balance Sheet Parenthetical (Parentheticals) (USD $)	Jun. 30, 2013	Dec. 31, 2012
Oil and Gas Property, Successful Effort Method, Accumulated Depreciation, Depletion and Amortization	$ 2,737,774,000	$ 2,376,170,000
Common Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01
Common Stock, Shares Authorized	200,000,000	200,000,000
Common Stock, Shares, Issued	66,412,394	66,245,816
Common Stock, Shares, Outstanding	66,389,982	66,195,235
Treasury Stock, Shares	22,412	50,581
Assets Held-for-sale [Member]
Oil and Gas Property, Successful Effort Method, Accumulated Depreciation, Depletion and Amortization	55,348,000	20,676,000
Other Capitalized Property Plant and Equipment [Member]
Oil and Gas Property, Successful Effort Method, Accumulated Depreciation, Depletion and Amortization	$ 25,542,000	$ 22,442,000
6.625% Senior Notes Due 2019 [Member]
Interest rate on notes (as a percent)	6.625%	6.625%
6.50% Senior Notes Due 2021 [Member]
Interest rate on notes (as a percent)	6.50%	6.50%
6.50% Senior Notes Due 2023 [Member]
Interest rate on notes (as a percent)	6.50%	6.50%
5.0% Senior Notes Due 2024 [Member]
Interest rate on notes (as a percent)	5.00%	5.00%