Stock-based compensation - Summary of Information about Restricted Stock Units Outstanding (Details) - Restricted stock units $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares shares
Number of Shares
Vested and expected to vest (in shares) | shares	2,166,565
Weighted Average Exercise Price
Vested and expected to vest (in USD per share) | $ / shares	$ 0.00
Weighted Average Remaining Contractual Term (Years)
Vested and expected to vest	1 year 3 months 4 days
Aggregate Intrinsic Value
Vested and expected to vest | $	$ 14,039,340