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Statements of Changes in Partners' Capital [Parenthetical] - shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Addition of partnership units 3,150,000 3,600,000 6,800,000
Redemption of partnership units 5,150,000 3,150,000 1,900,000