Marketable securities (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Marketable securities
Amortized Cost	$ 24,497		$ 98,774
Gross Unrealized Losses	(42)		(149)
Marketable securities	$ 24,455		98,625
Number of securities in an unrealized loss position | security	7
Aggregate fair value of securities in an unrealized loss position for less than 12 months	$ 24,455
Securities in an unrealized loss position for more than 12 months	0
Realized gains on available-for-sale securities	0	$ 0
Realized losses on available-for-sale securities	0	$ 0
U.S. Treasuries
Marketable securities
Amortized Cost	24,497		62,777
Gross Unrealized Losses	(42)		(137)
Marketable securities	$ 24,455		62,640
Commercial paper
Marketable securities
Amortized Cost			24,931
Marketable securities			24,931
Corporate bonds
Marketable securities
Amortized Cost			11,066
Gross Unrealized Losses			(12)
Marketable securities			$ 11,054