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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 |

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