Financial Instruments, Off-Balance Sheet Risks and Contingencies (Additional Information) (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
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	$ 206,860,594
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	$ 1,207,272,631

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.

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

Name:	us-gaap_Deposits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant