Cash, cash equivalents, short-term investments and restricted cash - Summary of Unrealized Gains and Losses (Details)	3 Months Ended
Mar. 31, 2020 USD ($) investment	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Impairment losses on marketable securities	$ 0	$ 0
Number of marketable securities in an unrealized loss position \| investment	0
Money market funds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 7,662,000		$ 7,638,000
Gross Unrealized Gains	0		0
Gross Unrealized Loss	0		0
Fair Value	7,662,000		7,638,000
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 7,662,000		$ 7,638,000

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).

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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).

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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).

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Amount, before tax, of other-than-temporary impairment (OTTI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), recognized in other comprehensive income (OCI).

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