Fair value of financial instruments and marketable securities - Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Marketable securities on the balance sheet
Total Marketable securities, Less Than 12 Months	$ 27,472	$ 157,352
Total Marketable securities, More Than 12 Months	0	15,993
Commercial paper
Marketable securities on the balance sheet
Total Marketable securities, Less Than 12 Months	0	12,966
Total Marketable securities, More Than 12 Months	0	0
Corporate debt securities
Marketable securities on the balance sheet
Total Marketable securities, Less Than 12 Months	27,472	137,196
Total Marketable securities, More Than 12 Months	0	15,993
Government obligations
Marketable securities on the balance sheet
Total Marketable securities, Less Than 12 Months	0	7,190
Total Marketable securities, More Than 12 Months	$ 0	$ 0