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Cash, cash equivalents, short-term investments and restricted cash - Summary of unrealized gains and losses (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) investment	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) investment	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Debt Securities, Available-for-sale [Line Items]
Impairment losses on marketable securities	$ 0	$ 0	$ 0	$ 0
Number of marketable securities in an unrealized loss position \| investment	0		0
Money market funds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 7,567,000		$ 7,567,000		$ 7,481,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Loss	0		0		0
Fair Value	7,567,000		7,567,000		7,481,000
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 7,567,000		$ 7,567,000		$ 7,481,000