Cash, cash equivalents, and available for sale marketable securities (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Cash and cash equivalents	$ 42,142	$ 31,530
Total marketable securities, amortized cost	138,724	138,780
Total money market funds and marketable securities	180,066	169,249
Fair Value
Cash and cash equivalents	42,142	31,530
Total marketable securities	138,259	138,622
Total money market funds and marketable securities	179,601	169,091
Unrealized Gains and Losses
Total marketable securities, unrealized losses	465	158
Total money market funds and marketable securities	$ 465	158
Maximum
Unrealized Gains and Losses
Debt securities, contractual maturity period	1 year
Money market funds
Amortized Cost
Cash and cash equivalents	$ 41,342	30,469
Fair Value
Cash and cash equivalents	41,342	30,469
U.S. Treasury notes
Amortized Cost
Marketable securities, amortized cost	137,504	137,560
Fair Value
Marketable debt securities, fair value	137,459	137,522
Unrealized Gains and Losses
Marketable securities, unrealized losses	45	38
Equity securities
Amortized Cost
Marketable equity securities, amortized cost	1,220	1,220
Fair Value
Marketable equity securities, fair value	800	1,100
Unrealized Gains and Losses
Equity securitiess, unrealized losses	$ 420	$ 120