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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Per Share Operating Performance:    
Net asset value, beginning of period $ 24.35 $ 23.34 [1]
Total income (loss) (0.85) (0.73)
Total expenses (0.08) (0.07)
Net increase (decrease) in net asset value (0.93) (0.80)
Net asset value, end of period $ 23.42 $ 22.54
Total Return (3.82%) (3.43%)
Ratios to Average Net Assets    
Total income (loss) 0.11% 2.85%
Management fees [2] 0.60% 0.60%
Expenses excluding management fees [2] 0.78% 0.70%
Net income (loss) (0.23%) 2.53%
[1] On January 4, 2018, there was a 1-for-4 reverse share split. Historical shares outstanding, net asset value per share and market value per share have been adjusted for the period shown to reflect the 1-for-4 reverse share split on a retroactive basis.
[2] Annualized.