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FAIR VALUE MEASUREMENTS (Details 2) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Reconciliation of changes in the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy    
Balance at beginning of period $ 41,159 $ 195,127
Total Gains or (Losses) (Realized or Unrealized):    
Included in Earnings 13,328 57,038
Included in Other Comprehensive Income (71,058) 22,692
Settlements (13,328) (56,300)
Transfers In and/or Out of Level 3   385
Balance at end of period (29,899) 218,942
Natural Gas Revenues
   
Derivative disclosures    
Unrealized gain (loss) included in natural gas revenues $ 0 $ 42