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Fair value measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Fair value measurements
Marketable securities	$ 17,976	$ 37,439
Transfers between fair value measurement level 1 to level 2	0		$ 0
Transfers between fair value measurement level 2 to level 1	0		$ 0
Recurring basis
Fair value measurements
Total assets regularly measured and carried at fair value	17,976	37,439
Recurring basis \| Commercial paper
Fair value measurements
Marketable securities	5,983	11,940
Recurring basis \| Corporate bonds
Fair value measurements
Marketable securities	11,993	12,010
Recurring basis \| U.S. Treasuries
Fair value measurements
Marketable securities		13,489
Recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value measurements
Total assets regularly measured and carried at fair value		13,489
Recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| U.S. Treasuries
Fair value measurements
Marketable securities		13,489
Recurring basis \| Significant Other Observable Inputs (Level 2)
Fair value measurements
Total assets regularly measured and carried at fair value	17,976	23,950
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Commercial paper
Fair value measurements
Marketable securities	5,983	11,940
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Corporate bonds
Fair value measurements
Marketable securities	$ 11,993	$ 12,010