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Fair Value Measurements (Fair Value of Liabilities Measured on Recurring Basis) (Details) (USD $)
In Thousands, unless otherwise specified
 6 Months Ended
Jun. 30, 2012
Dec. 31, 2011
Jun. 30, 2012
Fair Value on a recurring basis [Member]
Level 1 [Member]
Dec. 31, 2011
Fair Value on a recurring basis [Member]
Level 1 [Member]
Jun. 30, 2012
Fair Value on a recurring basis [Member]
Level 2 [Member]
Dec. 31, 2011
Fair Value on a recurring basis [Member]
Level 2 [Member]
Jun. 30, 2012
Fair Value on a recurring basis [Member]
Level 3 [Member]
Dec. 31, 2011
Fair Value on a recurring basis [Member]
Level 3 [Member]
Jun. 30, 2012
Fair Value on a recurring basis [Member]
Estimate of Fair Value, Fair Value Disclosure [Member]
Dec. 31, 2011
Fair Value on a recurring basis [Member]
Estimate of Fair Value, Fair Value Disclosure [Member]
Jun. 30, 2012
Fair Value on a nonrecurring basis [Member]
Level 3 [Member]
Jun. 30, 2012
Current assets - Fair market value of derivatives [Member]
Fair Value on a recurring basis [Member]
Level 1 [Member]
Dec. 31, 2011
Current assets - Fair market value of derivatives [Member]
Fair Value on a recurring basis [Member]
Level 1 [Member]
Jun. 30, 2012
Current assets - Fair market value of derivatives [Member]
Fair Value on a recurring basis [Member]
Level 2 [Member]
Dec. 31, 2011
Current assets - Fair market value of derivatives [Member]
Fair Value on a recurring basis [Member]
Level 2 [Member]
Jun. 30, 2012
Current assets - Fair market value of derivatives [Member]
Fair Value on a recurring basis [Member]
Level 3 [Member]
Dec. 31, 2011
Current assets - Fair market value of derivatives [Member]
Fair Value on a recurring basis [Member]
Level 3 [Member]
Jun. 30, 2012
Current assets - Fair market value of derivatives [Member]
Fair Value on a recurring basis [Member]
Estimate of Fair Value, Fair Value Disclosure [Member]
Dec. 31, 2011
Current assets - Fair market value of derivatives [Member]
Fair Value on a recurring basis [Member]
Estimate of Fair Value, Fair Value Disclosure [Member]
Jun. 30, 2012
Other long-term assets [Member]
Fair Value on a recurring basis [Member]
Level 1 [Member]
Dec. 31, 2011
Other long-term assets [Member]
Fair Value on a recurring basis [Member]
Level 1 [Member]
Jun. 30, 2012
Other long-term assets [Member]
Fair Value on a recurring basis [Member]
Level 2 [Member]
Dec. 31, 2011
Other long-term assets [Member]
Fair Value on a recurring basis [Member]
Level 2 [Member]
Jun. 30, 2012
Other long-term assets [Member]
Fair Value on a recurring basis [Member]
Level 3 [Member]
Dec. 31, 2011
Other long-term assets [Member]
Fair Value on a recurring basis [Member]
Level 3 [Member]
Jun. 30, 2012
Other long-term assets [Member]
Fair Value on a recurring basis [Member]
Estimate of Fair Value, Fair Value Disclosure [Member]
Dec. 31, 2011
Other long-term assets [Member]
Fair Value on a recurring basis [Member]
Estimate of Fair Value, Fair Value Disclosure [Member]
Jun. 30, 2012
Current liabilities - Fair market value of derivatives [Member]
Fair Value on a recurring basis [Member]
Level 1 [Member]
Jun. 30, 2012
Current liabilities - Fair market value of derivatives [Member]
Fair Value on a recurring basis [Member]
Level 2 [Member]
Jun. 30, 2012
Current liabilities - Fair market value of derivatives [Member]
Fair Value on a recurring basis [Member]
Level 3 [Member]
Jun. 30, 2012
Current liabilities - Fair market value of derivatives [Member]
Fair Value on a recurring basis [Member]
Estimate of Fair Value, Fair Value Disclosure [Member]
Jun. 30, 2012
Other long-term liabilities [Member]
Fair Value on a recurring basis [Member]
Level 1 [Member]
Jun. 30, 2012
Other long-term liabilities [Member]
Fair Value on a recurring basis [Member]
Level 2 [Member]
Jun. 30, 2012
Other long-term liabilities [Member]
Fair Value on a recurring basis [Member]
Level 3 [Member]
Jun. 30, 2012
Other long-term liabilities [Member]
Fair Value on a recurring basis [Member]
Estimate of Fair Value, Fair Value Disclosure [Member]
Derivative Assets                                                                      
Derivative Assets, Current $ 4,645 $ 2,499                   $ 0    $ 4,645 $ 2,499 $ 0    $ 4,645 $ 2,499                                
Derivative Assets, Noncurrent                                       0    1,982    0    1,982                   
Derivative Liabilities                                                                      
Derivative Liabilities, Current                                                       0 0 0 0        
Derivative Liabilities, Noncurrent                                                               0 350 0 350
Derivative Assets (Liabilities), at Fair Value, Net     0    6,277 2,499 0    6,277 2,499                                                  
Asset Retirement Obligation, Fair Value Disclosure                     $ 0