Investments in marketable securities Investments in marketable securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Cash Equivalents [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 4,961	$ 7,116
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	0
Investments in marketable securities, current	4,961	7,116
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Cash Equivalents [Member] \| Money Market Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	4,961	4,106
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	0
Investments in marketable securities, current	4,961	4,106
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Cash Equivalents [Member] \| US Government Agency Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	0	1,511
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	0
Investments in marketable securities, current	0	1,511
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Cash Equivalents [Member] \| US Treasury Bill Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	0	1,499
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	0
Investments in marketable securities, current	0	1,499
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Marketable Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	61,437	59,035
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	253	4
Investments in marketable securities, current	61,690	59,035
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	4
Marketable Securities [Member] \| Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Investments in marketable securities, current	58,475
Marketable Securities [Member] \| US Government Agency Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	25,214	19,863
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	88	2
Investments in marketable securities, current	25,302	19,864
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	1
Marketable Securities [Member] \| US Treasury Bill Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	9,974	15,926
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	24	2
Investments in marketable securities, current	9,998	15,927
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	1
Marketable Securities [Member] \| US Government Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	26,249	23,246
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	141	0
Investments in marketable securities, current	26,390	23,244
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 0	$ 2

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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Amortized cost 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 50
-Paragraph 2
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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)
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