Stockholders' equity - Warrants to Purchase Shares of Common Stock (Details) - $ / shares	3 Months Ended
Mar. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of Common Stock Subject to Warrants	10,506,524	10,506,524
August 2007 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 7.91
Expiration	2017-08
Shares of Common Stock Subject to Warrants	8,921	8,921
March 2008 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 790.54
Expiration	2018-03
Shares of Common Stock Subject to Warrants	46	46
November 2009 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 7.91
Expiration	2019-11
Shares of Common Stock Subject to Warrants	6,674	6,674
January 2010 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 7.91
Expiration	2020-01
Shares of Common Stock Subject to Warrants	6,674	6,674
March 2010 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 7.91
Expiration	2020-03
Shares of Common Stock Subject to Warrants	1,277	1,277
November 2011 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 7.91
Expiration	2021-11
Shares of Common Stock Subject to Warrants	5,213	5,213
December 2011 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 7.91
Expiration	2021-12
Shares of Common Stock Subject to Warrants	664	664
March 2012 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 109.90
Expiration	2019-03
Shares of Common Stock Subject to Warrants	4,125	4,125
February 2015 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 6.60
Expiration	2025-02
Shares of Common Stock Subject to Warrants	225,011	225,011
May 2015 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 6.60
Expiration	2020-05
Shares of Common Stock Subject to Warrants	3,457,750	3,457,750
May 2016 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 1.31
Expiration	2021-05
Shares of Common Stock Subject to Warrants	4,739,348	4,739,348
June 2016 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 1.31
Expiration	2021-05
Shares of Common Stock Subject to Warrants	2,050,821	2,050,821

Share-based Compensation Arrangement by Share-based Payment Award, Expiration Year and Month.

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- Definition

Exercise price per share or per unit of warrants or rights outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(i)(4))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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- Definition

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.

+ References

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- Definition

Number of equity instruments other than options outstanding, including both vested and non-vested instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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