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Short-term and Long-term investments - Composition of Short-term and Long-term Investments (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Short-term investments	$ 148,809,539us-gaap_ShortTermInvestments	$ 76,593,214us-gaap_ShortTermInvestments
Long-term investments	205,265,042us-gaap_LongTermInvestments	45,719,737us-gaap_LongTermInvestments
Time Deposits [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Short-term investments	58,475,057us-gaap_ShortTermInvestments / us-gaap_InvestmentTypeAxis = us-gaap_BankTimeDepositsMember	66,677,156us-gaap_ShortTermInvestments / us-gaap_InvestmentTypeAxis = us-gaap_BankTimeDepositsMember
Sovereign Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Short-term investments	4,721,440us-gaap_ShortTermInvestments / us-gaap_InvestmentTypeAxis = us-gaap_SovereignDebtSecuritiesMember	5,520,193us-gaap_ShortTermInvestments / us-gaap_InvestmentTypeAxis = us-gaap_SovereignDebtSecuritiesMember
Long-term investments	44,428,490us-gaap_LongTermInvestments / us-gaap_InvestmentTypeAxis = us-gaap_SovereignDebtSecuritiesMember	24,875,697us-gaap_LongTermInvestments / us-gaap_InvestmentTypeAxis = us-gaap_SovereignDebtSecuritiesMember
Corporate Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Short-term investments	81,810,731us-gaap_ShortTermInvestments / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember	4,395,865us-gaap_ShortTermInvestments / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember
Long-term investments	156,832,560us-gaap_LongTermInvestments / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember	20,844,040us-gaap_LongTermInvestments / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember
Certificates of Deposit [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Short-term investments	3,802,311us-gaap_ShortTermInvestments / us-gaap_InvestmentTypeAxis = us-gaap_CertificatesOfDepositMember
Long-term investments	$ 4,003,992us-gaap_LongTermInvestments / us-gaap_InvestmentTypeAxis = us-gaap_CertificatesOfDepositMember

X

- Definition

The total amount of investments that are intended to be held for an extended period of time (longer than one operating cycle).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.12)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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- Details

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X

- Definition

Investments which are intended to be sold in the short term (usually less than one year or the normal operating cycle, whichever is longer) including trading securities, available-for-sale securities, held-to-maturity securities, and other short-term investments not otherwise listed in the existing taxonomy.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Subparagraph g

-Article 7

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