Cash, cash equivalents and short-term investments - Summary of cost and estimated fair value of short-term investments (Details) $ in Thousands	Mar. 31, 2018 USD ($)
Amortized Cost
Schedule of Available-for-sale Securities [Line Items]
Due within one year	$ 33,956
Due between one and two years	26,349
Due between two and three years	11,391
Total	71,696
Estimated Fair Value
Schedule of Available-for-sale Securities [Line Items]
Due within one year	33,834
Due between one and two years	26,051
Due between two and three years	11,201
Total	$ 71,086