Investments - Investments in fixed income securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 41,020	$ 60,156
Gains		1
Losses	(2)	(4)
Fair Value	41,018	60,153
Corporate debt securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	930	6,556
Losses		(1)
Fair Value	930	6,555
U.S. Treasuries
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	3,897
Fair Value	3,897
Commercial paper
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	36,193	53,600
Gains		1
Losses	(2)	(3)
Fair Value	$ 36,191	$ 53,598

Amount of investment in debt security measured at amortized cost (held-to-maturity).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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