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Balance Sheet Parenthetical (Parentheticals) (USD $)
Mar. 31, 2013
Dec. 31, 2012
Oil and Gas Property, Successful Effort Method, Accumulated Depreciation, Depletion and Amortization $ 2,564,865,000 $ 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 66,300,003 66,245,816
Common Stock, Shares, Outstanding 66,249,422 66,195,235
Treasury Stock, Shares 50,581 50,581
Assets Held-for-sale [Member]
    
Oil and Gas Property, Successful Effort Method, Accumulated Depreciation, Depletion and Amortization 21,305,000 20,676,000
Other Capitalized Property Plant and Equipment [Member]
    
Oil and Gas Property, Successful Effort Method, Accumulated Depreciation, Depletion and Amortization $ 23,968,000 $ 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%