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

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- 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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- 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 + Details
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