Fair value measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Marketable securities:
Available-for-sale securities, Debt Securities, Current, Total	$ 296,629	$ 39,130
Recurring basis
Marketable securities:
Available-for-sale securities, Debt Securities, Current, Total	296,629
Recurring basis | Corporate debt securities
Marketable securities:
Available-for-sale securities, Debt Securities, Current, Total	288,632
Recurring basis | Agency bonds
Marketable securities:
Available-for-sale securities, Debt Securities, Current, Total	5,997
Recurring basis | Treasury bills
Marketable securities:
Available-for-sale securities, Debt Securities, Current, Total	2,000
Recurring basis | Level 1
Marketable securities:
Available-for-sale securities, Debt Securities, Current, Total	288,632
Recurring basis | Level 1 | Corporate debt securities
Marketable securities:
Available-for-sale securities, Debt Securities, Current, Total	288,632
Recurring basis | Level 2
Marketable securities:
Available-for-sale securities, Debt Securities, Current, Total	7,997
Recurring basis | Level 2 | Agency bonds
Marketable securities:
Available-for-sale securities, Debt Securities, Current, Total	5,997
Recurring basis | Level 2 | Treasury bills
Marketable securities:
Available-for-sale securities, Debt Securities, Current, Total	$ 2,000