Cash, cash equivalents and short-term investments - Summary of cost and estimated fair value of short-term investments (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Amortized Cost
Schedule of Available-for-sale Securities [Line Items]
Due within one year	$ 32,846
Due between one and two years	28,308
Due between two and three years	15,922
Total	77,076
Estimated Fair Value
Schedule of Available-for-sale Securities [Line Items]
Due within one year	32,801
Due between one and two years	28,156
Due between two and three years	15,816
Total	$ 76,773