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Derivatives (Open Commodity Derivative Contracts) (Detail)
6 Months Ended
Jun. 30, 2012
Crude Oil Exchange-Traded [Member]
 
Derivative [Line Items]  
Percentage of derivative contracts expiring in the period 99.10%
Derivative contract expiration date Sep. 30, 2012
Refined Products Exchange-Traded [Member]
 
Derivative [Line Items]  
Percentage of derivative contracts expiring in the period 100.00%
Derivative contract expiration date Sep. 30, 2012
Long [Member] | Crude Oil Exchange-Traded [Member]
 
Derivative [Line Items]  
Notional Contracts (in thousands of Total Barrels) 14,023 [1]
Long [Member] | Refined Products Exchange-Traded [Member]
 
Derivative [Line Items]  
Notional Contracts (in thousands of Total Barrels) 3,124 [2]
Short [Member] | Crude Oil Exchange-Traded [Member]
 
Derivative [Line Items]  
Notional Contracts (in thousands of Total Barrels) (26,774) [1]
Short [Member] | Refined Products Exchange-Traded [Member]
 
Derivative [Line Items]  
Notional Contracts (in thousands of Total Barrels) (7,062) [2]
[1] 99.1 percent of these contracts expire in the third quarter of 2012.
[2] 100 percent of these contracts expire in the third quarter of 2012.