Statements Of Financial Condition (USD $)	Dec. 31, 2012	Dec. 31, 2011
Assets
United States Treasury Obligations, at fair value (cost $277,480,954 and $437,494,687 respectively)	$ 277,495,828	$ 437,494,520
Cash held by broker	85,390,111	65,165,354
Net unrealized appreciation (depreciation) on futures contracts	(8,628,985)	(48,362,065)
Deposits with broker	354,256,954	454,297,809
Receivable for securities sold		10,999,933
Total assets	354,256,954	465,297,742
Liabilities
Payable for shares redeemed		10,813,002
Management fee payable	234,540	329,420
Brokerage fee payable	406	7,102
Total liabilities	234,946	11,149,524
Commitments and Contingencies (Note 9)
Equity
Shareholders' equity-General Shares	2,284	2,163
Shareholders' equity-Shares	354,019,724	454,146,055
Total shareholders' equity	354,022,008	454,148,218
Total liabilities and equity	$ 354,256,954	$ 465,297,742
General Shares outstanding	40	40
Shares outstanding	6,200,000	8,400,000
Net asset value per share
General shares	57.10	54.08
Shares	57.10	54.07