Summary of Significant Accounting Policies (Activity in Redeemable Capital Shares) (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012
Temporary Equity [Line Items]
Redeemable capital shares, prescribed aggregation	50,000	50,000
Period for Share settlement, days		3 days
Beginning balance, Shares	597,450,000	563,850,000
Shares issued, Shares	79,700,000	141,100,000
Shares redeemed, Shares	(13,050,000)	(40,850,000)
Ending balance, Shares	664,100,000	664,100,000
Beginning balance, Amount	$ 9,300,207	$ 8,416,801
Shares issued, Amount	1,303,832	2,302,662
Shares redeemed, Amount	(202,641)	(633,180)
Redemption value adjustment, Amount	1,077,175	1,392,290
Ending balance, Amount	$ 11,478,573	$ 11,478,573