Other balance sheet items - Schedule of Changes in Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance at beginning of period	$ (785)	$ (1,061)	$ (1,017)
Additions	(338)	(295)	(545)
Deductions	530	571	501
Balance at end of period	$ (593)	$ (785)	$ (1,061)