Fair Value Measurements - Recurring (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Dec. 31, 2019
Fair Value Measurements
Cash and cash equivalents	$ 18,605	$ 95,938
Recurring \| Embedded conversion option \| 2023 Notes
Fair Value Measurements
Embedded features	336	664
Recurring \| Embedded conversion option \| 2025 Notes
Fair Value Measurements
Embedded features	15,560	25,543
Recurring \| Money market funds
Fair Value Measurements
Cash and cash equivalents	1,501	37,769
Recurring \| Commercial
Fair Value Measurements
Cash and cash equivalents	4,797	13,870
Recurring \| Corporate bonds
Fair Value Measurements
Marketable securities		6,256
Recurring \| Level 1 \| Money market funds
Fair Value Measurements
Cash and cash equivalents	1,501	37,769
Recurring \| Level 2 \| Embedded conversion option \| 2025 Notes
Fair Value Measurements
Embedded features	15,560	25,543
Recurring \| Level 2 \| Commercial
Fair Value Measurements
Cash and cash equivalents	4,797	13,870
Recurring \| Level 2 \| Corporate bonds
Fair Value Measurements
Marketable securities		6,256
Recurring \| Level 3 \| Embedded conversion option
Reconciliation of items measured at fair value on a recurring basis that used significant unobservable inputs (Level 3)
Balance at the beginning of the period	664
Balance at the end of the period	336
Recurring \| Level 3 \| Embedded conversion option \| 2023 Notes
Fair Value Measurements
Embedded features	336	$ 664
Reconciliation of items measured at fair value on a recurring basis that used significant unobservable inputs (Level 3)
Change in derivative liabilities	$ (328)

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled after one year or the normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Amount of gain (loss) recognized in income from liability measured at fair value on recurring basis using unobservable input (level 3).

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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.

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