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Fair Value Measurements (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Assets:    
Proved Oil and Gas Properties $ 5,670,183 $ 5,401,684
Unproved oil and gas properties 173,215 175,287
Oil and gas properties held for sale net of accumulated depletion, depreciation and amortization of $21,305 in 2013 and 20,676 in 2012 33,340 33,620
Fair Value, Measurements, Recurring [Member] | Level 1
    
Liabilities    
Net Profits Plan Liability 0 [1] 0 [1]
Fair Value, Measurements, Recurring [Member] | Level 1 | Derivatives not designated as hedging instruments
    
Assets:    
Derivatives 0 [1] 0 [1]
Liabilities    
Derivatives 0 [1] 0 [1]
Fair Value, Measurements, Recurring [Member] | Level 2
    
Liabilities    
Net Profits Plan Liability 0 [1] 0 [1]
Fair Value, Measurements, Recurring [Member] | Level 2 | Derivatives not designated as hedging instruments
    
Assets:    
Derivatives 31,935 [1] 54,339 [1]
Liabilities    
Derivatives 35,505 [1] 15,644 [1]
Fair Value, Measurements, Recurring [Member] | Level 3
    
Liabilities    
Net Profits Plan Liability 76,902 [1] 78,827 [1]
Fair Value, Measurements, Recurring [Member] | Level 3 | Derivatives not designated as hedging instruments
    
Assets:    
Derivatives 0 [1] 0 [1]
Liabilities    
Derivatives 0 [1] 0 [1]
Fair Value, Measurements, Nonrecurring [Member] | Level 1
    
Assets:    
Proved Oil and Gas Properties   0 [2]
Unproved oil and gas properties   0 [2]
Oil and gas properties held for sale net of accumulated depletion, depreciation and amortization of $21,305 in 2013 and 20,676 in 2012 0 [2] 0 [2]
Fair Value, Measurements, Nonrecurring [Member] | Level 2
    
Assets:    
Proved Oil and Gas Properties   0 [2]
Unproved oil and gas properties   0 [2]
Oil and gas properties held for sale net of accumulated depletion, depreciation and amortization of $21,305 in 2013 and 20,676 in 2012 0 [2] 0 [2]
Fair Value, Measurements, Nonrecurring [Member] | Level 3
    
Assets:    
Proved Oil and Gas Properties   209,959 [2]
Unproved oil and gas properties   42,765 [2]
Oil and gas properties held for sale net of accumulated depletion, depreciation and amortization of $21,305 in 2013 and 20,676 in 2012 $ 15,095 [2] $ 16,527 [2]
[1] This represents a financial asset or liability that is measured at fair value on a recurring basis.
[2] This represents a non-financial asset or liability that is measured at fair value on a nonrecurring basis.