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Summary of Significant Accounting Policies (Activity in Redeemable Capital Shares) (Details) (USD $)
In Thousands, except Share data
 3 Months Ended 6 Months Ended
Jun. 30, 2011
Jun. 30, 2011
Summary of Significant Accounting Policies    
Redeemable capital shares, prescribed aggregation 50,000 50,000
Beginning balance, redeemable capital shares 366,800,000 359,200,000
Shares issued, shares 44,750,000 88,850,000
Shares redeemed, shares (97,150,000) (133,650,000)
Ending balance, redeemable capital shares 314,400,000 314,400,000
Beginning balance, redeemable capital shares value $ 13,557,591 $ 10,750,994
Shares issued, value 1,769,807 3,174,587
Shares redeemed, value (3,846,750) (4,903,925)
Redemption value adjustment (747,774) 1,711,218
Ending balance, redeemable capital shares value $ 10,732,874 $ 10,732,874