Fair value of financial instruments and marketable securities - Warrants and SARs (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2016	Dec. 31, 2015
Stock Appreciation Rights (SARs)
Changes in the fair value of warrant liability and SARs liability
Change in fair value	$ 140
Ending balance	$ 140
Assumption used to estimate the fair value of warrant liability and SARs liability by utilizing the Black-Scholes option-pricing model
Volatility (as a percent)	70.00%
Minimum \| Stock Appreciation Rights (SARs)
Assumption used to estimate the fair value of warrant liability and SARs liability by utilizing the Black-Scholes option-pricing model
Volatility (as a percent)	0.36%
Strike price (in dollars per share)	$ 6.76
Expected life	6 months 7 days
Maximum \| Stock Appreciation Rights (SARs)
Assumption used to estimate the fair value of warrant liability and SARs liability by utilizing the Black-Scholes option-pricing model
Risk-free interest rate (as a percent)	0.86%
Strike price (in dollars per share)	$ 30.86
Expected life	3 years 6 months 7 days
Common stock \| Stock Appreciation Rights (SARs)
Assumption used to estimate the fair value of warrant liability and SARs liability by utilizing the Black-Scholes option-pricing model
Fair value of shares (in dollars per share)	$ 7.02
Warrants
Changes in the fair value of warrant liability and SARs liability
Beginning balance	$ 48
Change in fair value	(47)
Ending balance	$ 1	$ 48
Warrants \| Minimum
Assumption used to estimate the fair value of warrant liability and SARs liability by utilizing the Black-Scholes option-pricing model
Volatility (as a percent)	75.00%	62.00%
Risk-free interest rate (as a percent)	0.45%	0.86%
Strike price (in dollars per share)	$ 128.00	$ 128.00
Expected life	11 months 16 days	1 year 6 months
Warrants \| Maximum
Assumption used to estimate the fair value of warrant liability and SARs liability by utilizing the Black-Scholes option-pricing model
Volatility (as a percent)	77.00%	70.00%
Risk-free interest rate (as a percent)	0.71%	1.54%
Strike price (in dollars per share)	$ 2,520.00	$ 2,520.00
Expected life	3 years 2 months 23 days	3 years 8 months 12 days
Warrants \| Common stock
Assumption used to estimate the fair value of warrant liability and SARs liability by utilizing the Black-Scholes option-pricing model
Fair value of shares (in dollars per share)	$ 7.02	$ 32.40

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- Definition

Agreed upon price for the exchange of the underlying asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Period the instrument, asset or liability is expected to be outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Measure of dispersion, in percentage terms (for instance, the standard deviation or variance), for a given stock price.

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Risk-free interest rate assumption used in valuing an instrument.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Amount of gain (loss) recognized on the income statement for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Price of a single share of a number of saleable stocks of a company.

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