Fair value of financial instruments Schedule of assets and liabilities measured at fair value on a recurring basis, by level (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets, Fair Value Disclosure [Abstract]
Fair value	$ 25,704	$ 34,276
Fair value	204,164	186,894
U.S. government agency securities
Assets, Fair Value Disclosure [Abstract]
Fair value	3,000	3,325
U.S. Treasury securities
Assets, Fair Value Disclosure [Abstract]
Fair value		2,730
Corporate debt securities
Assets, Fair Value Disclosure [Abstract]
Fair value	188,310	166,759
Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	208,250	193,121
Fair Value, Measurements, Recurring \| Level 1
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	4,086	3,737
Fair Value, Measurements, Recurring \| Level 2
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	204,164	189,384
Fair Value, Measurements, Recurring \| U.S. government agency securities
Assets, Fair Value Disclosure [Abstract]
Fair value	3,000	3,325
Fair Value, Measurements, Recurring \| U.S. government agency securities \| Level 1
Assets, Fair Value Disclosure [Abstract]
Fair value	0	0
Fair Value, Measurements, Recurring \| U.S. government agency securities \| Level 2
Assets, Fair Value Disclosure [Abstract]
Fair value	3,000	3,325
Fair Value, Measurements, Recurring \| Corporate debt securities
Assets, Fair Value Disclosure [Abstract]
Fair value	188,310	166,759
Fair Value, Measurements, Recurring \| Corporate debt securities \| Level 1
Assets, Fair Value Disclosure [Abstract]
Fair value	0	0
Fair Value, Measurements, Recurring \| Corporate debt securities \| Level 2
Assets, Fair Value Disclosure [Abstract]
Fair value	188,310	166,759
Fair Value, Measurements, Recurring \| Money Market Funds [Member]
Assets, Fair Value Disclosure [Abstract]
Fair value	4,086	3,737
Fair Value, Measurements, Recurring \| Money Market Funds [Member] \| Level 1
Assets, Fair Value Disclosure [Abstract]
Fair value	4,086	3,737
Fair Value, Measurements, Recurring \| Money Market Funds [Member] \| Level 2
Assets, Fair Value Disclosure [Abstract]
Fair value	0	0
Fair Value, Measurements, Recurring \| Commercial paper
Assets, Fair Value Disclosure [Abstract]
Fair value	12,854	16,570
Fair Value, Measurements, Recurring \| Commercial paper \| Level 1
Assets, Fair Value Disclosure [Abstract]
Fair value	0	0
Fair Value, Measurements, Recurring \| Commercial paper \| Level 2
Assets, Fair Value Disclosure [Abstract]
Fair value	$ 12,854	$ 16,570

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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- Definition

Amount of 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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

Fair value portion 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.

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