Statements Of Financial Condition (Parenthetical) (USD $)	Sep. 30, 2012	Dec. 31, 2011
United States Treasury Obligations, cost	$ 1,629,756,268	$ 1,959,877,867
General shares, shares issued	40	40
General shares, shares outstanding	40	40
Limited shares, redeemable shares issued	65,000,000	70,600,000
Limited shares, redeemable shares outstanding	65,000,000	70,600,000