Stockholders' equity - Warrants to Purchase Shares of Common Stock (Details) - $ / shares	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of Common Stock Subject to Warrants	3,525,797	1,495,031
March 2008 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 19,763.50
Expiration	2018-03
Shares of Common Stock Subject to Warrants		2
November 2009 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 197.75
Expiration	2019-11
Shares of Common Stock Subject to Warrants	270	270
January 2010 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 197.75
Expiration	2020-01
Shares of Common Stock Subject to Warrants	270	270
March 2010 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 197.75
Expiration	2020-03
Shares of Common Stock Subject to Warrants	55	55
November 2011 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 197.75
Expiration	2021-11
Shares of Common Stock Subject to Warrants	212	212
December 2011 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 197.75
Expiration	2021-12
Shares of Common Stock Subject to Warrants	27	27
March 2012 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 2,747.50
Expiration	2019-03
Shares of Common Stock Subject to Warrants	165	165
February 2015 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 165.00
Expiration	2025-02
Shares of Common Stock Subject to Warrants	9,001	9,001
May 2015 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 165.00
Expiration	2020-05
Shares of Common Stock Subject to Warrants	138,310	138,310
May 2016 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 32.81
Expiration	2021-05
Shares of Common Stock Subject to Warrants	189,577	189,577
June 2016 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 32.81
Expiration	2021-05
Shares of Common Stock Subject to Warrants	82,035	82,035
June 2017 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 19.50
Expiration	2022-06
Shares of Common Stock Subject to Warrants	18,754	18,754
July 2017 [Member] | Exercise Price 17.25 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 17.25
Expiration	2022-07
Shares of Common Stock Subject to Warrants	6,350	6,350
July 2017 [Member] | Exercise Price 12.50 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 12.50
Expiration	2022-07
Shares of Common Stock Subject to Warrants	50,000	50,000
July 2017 [Member] | Exercise Price 10.63 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 10.63
Expiration	2022-07
Shares of Common Stock Subject to Warrants	1,000,003	1,000,003
February 2018 [Member] | Exercise Price 4.06 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 4.06
Expiration	2023-02
Shares of Common Stock Subject to Warrants	184,615
February 2018 [Member] | Exercise Price 3.25 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price	$ 3.25
Expiration	2023-02
Shares of Common Stock Subject to Warrants	1,846,153