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Financial Highlights (Per Unit Performance Data and Other Supplemental Financial Data) (Detail) (USD $)
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Apr. 10, 2006
Per Share Operating Performance:      
Net asset value, beginning of year $ 20.15uso_NetAssetValuePerUnit $ 35.23uso_NetAssetValuePerUnit $ 67.39uso_NetAssetValuePerUnit
Total income (loss) $ (3.25)uso_TotalIncomeLossPerUnit $ 1.45uso_TotalIncomeLossPerUnit  
Total expenses $ (0.03)uso_TotalExpensesPerUnit $ (0.07)uso_TotalExpensesPerUnit  
Net increase (decrease) in net asset value $ (3.28)uso_IncreaseDecreaseInNetAssetValuePerUnit $ 1.38uso_IncreaseDecreaseInNetAssetValuePerUnit  
Net asset value, end of year $ 16.87uso_NetAssetValuePerUnit $ 36.61uso_NetAssetValuePerUnit $ 67.39uso_NetAssetValuePerUnit
Total Return (16.28%)uso_RateOfReturnOnAverageAsset 3.92%uso_RateOfReturnOnAverageAsset  
Ratios to Average Net Assets      
Total income (loss) (11.82%)uso_PercentageOfTotalRevenue 4.57%uso_PercentageOfTotalRevenue  
Expenses excluding management fees 0.27%uso_ExpensesExcludingManagementFeePercentage [1] 0.34%uso_ExpensesExcludingManagementFeePercentage [1]  
Management fees 0.45%uso_ManagementIncentiveFeePercentage [1] 0.45%uso_ManagementIncentiveFeePercentage [1]  
Net income (loss) (12.00%)uso_PercentageOfConsolidatedNetIncomeLoss 4.37%uso_PercentageOfConsolidatedNetIncomeLoss  
[1] Annualized