Fair value of financial instruments and marketable securities - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017	Aug. 31, 2015
Financial assets and liabilities measured at fair value on recurring basis
Transfers of assets between Level 1 and Level 2 of the fair value measurement hierarchy	$ 0	$ 0
Realized gains	$ 0	$ 0
Minimum \| Warrants
Financial assets and liabilities measured at fair value on recurring basis
Volatility	54.00%	69.00%
Risk-free interest rate (as a percent)	2.18%	1.89%
Strike price (in dollars per share)	$ 128.00	$ 128.00
Expected life	1 year 4 months 6 days	1 year 7 months 6 days
Maximum \| Warrants
Financial assets and liabilities measured at fair value on recurring basis
Volatility	56.00%	69.00%
Risk-free interest rate (as a percent)	2.18%	1.89%
Strike price (in dollars per share)	$ 2,520.00	$ 2,520.00
Expected life	1 year 5 months 23 days	1 year 8 months 12 days
Common stock \| Warrants
Financial assets and liabilities measured at fair value on recurring basis
Fair value of shares (in dollars per share)	$ 27.06	$ 16.68
SARs \| Minimum
Financial assets and liabilities measured at fair value on recurring basis
Volatility	55.00%	31.00%
Risk-free interest rate (as a percent)	2.01%	1.28%
Strike price (in dollars per share)	$ 6.76	$ 6.76
Expected life	9 months 4 days	0 days
SARs \| Maximum
Financial assets and liabilities measured at fair value on recurring basis
Volatility	58.00%	70.00%
Risk-free interest rate (as a percent)	2.18%	1.89%
Strike price (in dollars per share)	$ 30.86	$ 30.86
Expected life	1 year 9 months 4 days	2 years
SARs \| Common stock
Financial assets and liabilities measured at fair value on recurring basis
Fair value of shares (in dollars per share)	$ 27.06	$ 16.68
3.00% Convertible senior notes due 2022 \| Convertible debt
Financial assets and liabilities measured at fair value on recurring basis
Principal	$ 150,000,000	$ 150,000,000	$ 150,000,000.0
Interest rate			3.00%
Fair value of convertible notes	$ 139,300,000	$ 115,700,000

Amount of transfers of assets measured on a recurring basis between Level 1 and Level 2 of the fair value hierarchy.

No definition available.

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- Definition

This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

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- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13476-108611

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ 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 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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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

No definition available.

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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 (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&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.

+ References

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- Definition

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

+ References

+ Details

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- Definition

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

+ References

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- Definition

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

+ References

No definition available.

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