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Financial Instruments, Off-Balance Sheet Risks and Contingencies (Additional Information) (Detail) (USD $) | Mar. 31, 2015 | Dec. 31, 2014
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Money Market Funds [Member] |  |
Loss Contingencies [Line Items] |  |
Deposits in domestic and foreign financial institutions, including cash investments in money market funds | $ 306,860,594 us-gaap_Deposits / us-gaap_StatementEquityComponentsAxis = us-gaap_MoneyMarketFundsMember | $ 206,860,594 us-gaap_Deposits / us-gaap_StatementEquityComponentsAxis = us-gaap_MoneyMarketFundsMember
US Treasury Securities [Member] |  |
Loss Contingencies [Line Items] |  |
Deposits in domestic and foreign financial institutions, including cash investments in money market funds | $ 2,662,854,637 us-gaap_Deposits / us-gaap_StatementEquityComponentsAxis = us-gaap_USTreasurySecuritiesMember | $ 1,207,272,631 us-gaap_Deposits / us-gaap_StatementEquityComponentsAxis = us-gaap_USTreasurySecuritiesMember

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- Definition The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.12) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 12 -Article 9 + Details
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