Fair value measurements - Assets measured on a recurring basis - (Details) $ in Thousands	Sep. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($) item
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 42,142	$ 31,530
Measurement Input, Risk-Free Interest Rate
Fair value assumptions
Warrants, measurement input \| item	0.029	0.020
Measurement Input, Expected Term
Fair value assumptions
Warrants, measurement input \| item	2.9	3.7
Measurement Input, Expected Volatility
Fair value assumptions
Warrants, measurement input \| item	1.120	1.035
Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	$ 138,259	$ 138,622
Total assets measured at fair value	179,957	169,660
Level 1 \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	138,259	138,622
Total assets measured at fair value	179,601	169,091
Level 2 \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	356	569
U.S. Treasury notes \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	137,459	137,522
U.S. Treasury notes \| Level 1 \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	137,459	137,522
Equity securities \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	800	1,100
Equity securities \| Level 1 \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	800	1,100
Warrants to purchase equity securities \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Warrants to purchase equity securities	356	569
Warrants to purchase equity securities \| Level 2 \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Warrants to purchase equity securities	356	569
Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	41,342	30,469
Money market funds \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	41,342	30,469
Money market funds \| Level 1 \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 41,342	$ 30,469

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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 4
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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Amount of investment in marketable security.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
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Value of outstanding derivative securities that permit the holder the right to purchase securities (usually equity) from the issuer at a specified price.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
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