Summary of Significant Accounting Policies (Activity in Redeemable Capital Shares) (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Temporary Equity [Line Items]
Redeemable capital shares, prescribed aggregation	50,000
Period for Share settlement, days	3 days
Beginning balance, Shares	563,850,000	386,950,000	260,000,000
Shares issued, Shares	200,400,000	239,050,000	138,950,000
Shares redeemed, Shares	(44,700,000)	(62,150,000)	(12,000,000)
Ending balance, Shares	719,550,000	563,850,000	386,950,000
Beginning balance, Amount	$ 8,416,801	$ 5,315,950	$ 2,792,233
Shares issued, Amount	3,303,091	3,660,287	1,748,964
Shares redeemed, Amount	(697,017)	(915,939)	(128,153)
Redemption value adjustment, Amount	622,423	356,503	902,906
Ending balance, Amount	$ 11,645,298	$ 8,416,801	$ 5,315,950