Other balance sheet items - Schedule of Changes in Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]
Trade receivables allowance, beginning of period	$ (1,061)	$ (1,017)	$ (682)
Additions	(295)	(545)	(938)
Deductions	571	501	603
Trade receivables allowance, end of period	$ (785)	$ (1,061)	$ (1,017)