Marketable securities (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) security	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Marketable securities
Amortized Cost	$ 75,767,000		$ 98,774,000
Gross Unrealized Losses	(162,000)		(149,000)
Marketable securities	$ 75,605,000		98,625,000
Number of securities in an unrealized loss position | security	19
Aggregate fair value of securities in an unrealized loss position for less than 12 months	$ 65,617,000
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	59,378,000		62,777,000
Gross Unrealized Losses	(153,000)		(137,000)
Marketable securities	59,225,000		62,640,000
Commercial paper
Marketable securities
Amortized Cost	9,989,000		24,931,000
Marketable securities	9,989,000		24,931,000
Corporate bonds
Marketable securities
Amortized Cost	6,400,000		11,066,000
Gross Unrealized Losses	(9,000)		(12,000)
Marketable securities	$ 6,391,000		$ 11,054,000