Fair value measurements (Details) - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Marketable securities:
Fair Value	$ 31,743,086	$ 38,658,933
Corporate debt securities
Marketable securities:
Fair Value	3,102,128	1,518,240
Commercial paper
Marketable securities:
Fair Value	4,594,668	6,081,202
US government agency securities
Marketable securities:
Fair Value	24,046,290	31,059,491
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	11,115,898	11,681,074
Marketable securities:
Total assets	42,858,984	50,340,007
Fair Value, Measurements, Recurring \| Corporate debt securities
Marketable securities:
Fair Value	3,102,128	1,518,240
Fair Value, Measurements, Recurring \| Commercial paper
Marketable securities:
Fair Value	4,594,668	6,081,202
Fair Value, Measurements, Recurring \| US government agency securities
Marketable securities:
Fair Value	24,046,290	31,059,491
Fair Value, Measurements, Recurring \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	11,115,898	11,681,074
Marketable securities:
Total assets	11,115,898	11,681,074
Fair Value, Measurements, Recurring \| Level 1 \| Corporate debt securities
Marketable securities:
Fair Value	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Commercial paper
Marketable securities:
Fair Value	0	0
Fair Value, Measurements, Recurring \| Level 1 \| US government agency securities
Marketable securities:
Fair Value	0	0
Fair Value, Measurements, Recurring \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Marketable securities:
Total assets	31,743,086	38,658,933
Fair Value, Measurements, Recurring \| Level 2 \| Corporate debt securities
Marketable securities:
Fair Value	3,102,128	1,518,240
Fair Value, Measurements, Recurring \| Level 2 \| Commercial paper
Marketable securities:
Fair Value	4,594,668	6,081,202
Fair Value, Measurements, Recurring \| Level 2 \| US government agency securities
Marketable securities:
Fair Value	24,046,290	31,059,491
Fair Value, Measurements, Recurring \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Marketable securities:
Total assets	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Corporate debt securities
Marketable securities:
Fair Value	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Commercial paper
Marketable securities:
Fair Value	0	0
Fair Value, Measurements, Recurring \| Level 3 \| US government agency securities
Marketable securities:
Fair Value	$ 0	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

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- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash
-URI http://asc.fasb.org/extlink&oid=6506951

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash Equivalents
-URI http://asc.fasb.org/extlink&oid=6507016

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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