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

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

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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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

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

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- Definition

Amount of realized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/recommendedDisclosureRef
-Publisher FASB
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- Definition

Value of input used to measure derivative liability.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
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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

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

+ References

No definition available.

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- Definition

Value of input used to measure outstanding warrant and right embodying unconditional obligation requiring redemption by transferring asset at specified or determinable date or upon event certain to occur.

+ References

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Period between issuance and expiration of outstanding warrant and right embodying unconditional obligation requiring redemption by transferring asset at specified or determinable date or upon event certain to occur, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Fair value of financial instruments and marketable securities - Narrative (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares	Apr. 08, 2018 $ / shares	Aug. 31, 2015 USD ($)
Financial assets and liabilities measured at fair value on recurring basis
Realized gain (loss) \| $	$ 0	$ 0
Transfers of assets between Level 1 and Level 2 of the fair value measurement hierarchy \| $	$ 0	$ 0
Fair value of shares (in dollars per share)			$ 27.04
Warrants
Financial assets and liabilities measured at fair value on recurring basis
Fair value of shares (in dollars per share)	$ 33.73	$ 16.68
Minimum \| Warrants
Financial assets and liabilities measured at fair value on recurring basis
Warrants, exercise price (in dollars per share)		128.00
Maximum \| Warrants
Financial assets and liabilities measured at fair value on recurring basis
Warrants, exercise price (in dollars per share)		2,520.00
SARs
Financial assets and liabilities measured at fair value on recurring basis
Fair value of shares (in dollars per share)	$ 33.73	$ 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 \| $	$ 148,200,000	$ 115,700,000
Measurement Input, Price Volatility \| Minimum \| Warrants
Financial assets and liabilities measured at fair value on recurring basis
Derivative liability, measurement input	0.54	0.69
Measurement Input, Price Volatility \| Maximum \| Warrants
Financial assets and liabilities measured at fair value on recurring basis
Derivative liability, measurement input	0.53	0.69
Measurement Input, Price Volatility \| SARs \| Minimum
Financial assets and liabilities measured at fair value on recurring basis
Derivative liability, measurement input	0.55	0.31
Measurement Input, Price Volatility \| SARs \| Maximum
Financial assets and liabilities measured at fair value on recurring basis
Derivative liability, measurement input	0.56	0.70
Measurement Input, Risk Free Interest Rate \| Minimum \| Warrants
Financial assets and liabilities measured at fair value on recurring basis
Derivative liability, measurement input	0.0243	0.0189
Measurement Input, Risk Free Interest Rate \| Maximum \| Warrants
Financial assets and liabilities measured at fair value on recurring basis
Derivative liability, measurement input	0.0243	0.0189
Measurement Input, Risk Free Interest Rate \| SARs \| Minimum
Financial assets and liabilities measured at fair value on recurring basis
Derivative liability, measurement input	0.0222	0.0128
Measurement Input, Risk Free Interest Rate \| SARs \| Maximum
Financial assets and liabilities measured at fair value on recurring basis
Derivative liability, measurement input	0.0243	0.0189
Measurement Input, Strike Price \| Minimum \| Warrants
Financial assets and liabilities measured at fair value on recurring basis
Warrants, exercise price (in dollars per share)	128.00
Measurement Input, Strike Price \| Maximum \| Warrants
Financial assets and liabilities measured at fair value on recurring basis
Warrants, exercise price (in dollars per share)	2,520.00
Measurement Input, Strike Price \| SARs \| Minimum
Financial assets and liabilities measured at fair value on recurring basis
Warrants, exercise price (in dollars per share)	6.76	6.76
Measurement Input, Strike Price \| SARs \| Maximum
Financial assets and liabilities measured at fair value on recurring basis
Warrants, exercise price (in dollars per share)	30.86	30.86
Measurement Input, Expected Term \| Minimum \| Warrants
Financial assets and liabilities measured at fair value on recurring basis
Warrants, term	1 year 2 months 23 days	1 year 7 months 6 days
Measurement Input, Expected Term \| Maximum \| Warrants
Financial assets and liabilities measured at fair value on recurring basis
Warrants, term	1 year 1 month 6 days	1 year 8 months 12 days
Measurement Input, Expected Term \| SARs \| Minimum
Financial assets and liabilities measured at fair value on recurring basis
Warrants, term	6 months 7 days	0 days
Measurement Input, Expected Term \| SARs \| Maximum
Financial assets and liabilities measured at fair value on recurring basis
Warrants, term	1 year 6 months 7 days	2 years