Fair value measurements - Assets measured on a recurring basis - (Details) $ in Thousands	Jun. 30, 2020 USD ($) item	Dec. 31, 2019 USD ($) item
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 144,321	$ 86,042
Measurement Input, Risk-Free Interest Rate
Fair value assumptions
Warrants, measurement input \| item	0.002	0.016
Measurement Input, Expected Term
Fair value assumptions
Warrants, measurement input \| item	1.2	1.7
Measurement Input, Expected Volatility
Fair value assumptions
Warrants, measurement input \| item	0.691	0.716
Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	$ 86,771	$ 197,411
Total assets measured at fair value	224,724	276,268
Level 1 \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	86,771	197,411
Total assets measured at fair value	224,526	275,714
Level 2 \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	198	554
U.S. Treasury notes \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	85,349	195,491
U.S. Treasury notes \| Level 1 \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	85,349	195,491
Equity securities \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	1,422	1,920
Equity securities \| Level 1 \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	1,422	1,920
Warrants \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Warrants to purchase equity securities	198	554
Warrants \| Level 2 \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Warrants to purchase equity securities	198	554
Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	137,755	78,303
Money market funds \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	137,755	78,303
Money market funds \| Level 1 \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 137,755	$ 78,303

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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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 asset or other 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. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Amount of investment in marketable security.

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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.

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