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FEES PAID BY THE FUND AND RELATED PARTY TRANSACTIONS - Additional Information (Details) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2019
Fees Paid and Related Party Transactions [Line Items]          
Percentage of management fee in average daily total net assets     0.60%    
Licensing fee incurred $ 1,992 $ 3,010 $ 6,396 $ 9,357  
Estimated directors' fees and expenses $ 3,393 $ 3,642 $ 9,398 $ 10,456  
Scenario Forecast          
Fees Paid and Related Party Transactions [Line Items]          
Estimated investor tax reporting cost         $ 120,000
Estimated directors' fees and expenses         13,000
USL and the Related Public Funds, other than USCI, USAG, CPER and USMI | Scenario Forecast          
Fees Paid and Related Party Transactions [Line Items]          
Estimated directors' fees and expenses         $ 543,500
On and after October 20, 2011          
Fees Paid and Related Party Transactions [Line Items]          
Fee percentage     0.015%