Cash, cash equivalents and short-term investments - Summary of cost and estimated fair value of short-term investments (Details) $ in Thousands	Jun. 30, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 70,493
Amortized Cost
Debt Securities, Available-for-sale [Line Items]
Due within one year	31,526
Due between one and two years	23,608
Due between two and three years	15,359
Amortized Cost	70,493
Estimated Fair Value
Debt Securities, Available-for-sale [Line Items]
Due within one year	31,512
Due between one and two years	23,774
Due between two and three years	15,568
Total	$ 70,854