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Financial Instruments (Details 6) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Reconciliation of changes in the fair value of derivatives classified as Level 3 in the fair value hierarchy    
Assets balance, beginning of period $ 10,774 $ 14,192
Total gains or losses:    
Non-cash amortization of option premium (21,757) (29,515)
Other amounts included in earnings 15,139 (9,946)
Included in accumulated other comprehensive loss 18,337 13,759
Purchases 8,557 11,151
Issuances (7,390)  
Settlements (5,596) 10,413
Assets balance, end of period 18,064 10,774
Change in unrealized (income) losses included in earnings related to instruments still held as of the end of the year (79) 2,133
Natural Gas options
   
Reconciliation of changes in the fair value of derivatives classified as Level 3 in the fair value hierarchy    
Assets balance, beginning of period   87
Total gains or losses:    
Non-cash amortization of option premium   (5,895)
Included in accumulated other comprehensive loss (56) 5,808
Purchases 404  
Assets balance, end of period 348  
Natural Gas Liquids
   
Reconciliation of changes in the fair value of derivatives classified as Level 3 in the fair value hierarchy    
Assets balance, beginning of period 5,471 8,349
Total gains or losses:    
Non-cash amortization of option premium (15,347) (15,671)
Other amounts included in earnings 14,818 (11,155)
Included in accumulated other comprehensive loss 16,216 4,184
Purchases 3,908 9,351
Issuances (7,390)  
Settlements (5,466) 10,413
Assets balance, end of period 12,210 5,471
Change in unrealized (income) losses included in earnings related to instruments still held as of the end of the year (293) 1,200
Crude Oil
   
Reconciliation of changes in the fair value of derivatives classified as Level 3 in the fair value hierarchy    
Assets balance, beginning of period 5,303 6,476
Total gains or losses:    
Non-cash amortization of option premium (6,410) (7,949)
Other amounts included in earnings 321 1,209
Included in accumulated other comprehensive loss 2,177 3,767
Purchases 4,245 1,800
Settlements (130)  
Assets balance, end of period 5,506 5,303
Change in unrealized (income) losses included in earnings related to instruments still held as of the end of the year $ 214 $ 933