Income taxes - Difference in Provision for Income Taxes from Amount Obtained by Applying U.S. Federal Statutory Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Federal statutory rate	21.00%	35.00%	35.00%
Tax at federal statutory rate	$ 8,869	$ 18,354	$ (1,185)
State taxes, net of federal benefit	36	2	(8)
Mandatory repatriation/Section 956	0	(5,718)	(19)
Nondeductible expenses	(371)	(67)	(727)
Stock-based compensation	(1,079)	(314)	(877)
Change in valuation allowance	(10,094)	16,273	(1,455)
Research and development	914	851	1,175
Foreign rate differences	(697)	(2,819)	(1,215)
Foreign tax credit	49	144	127
Change in prior year deferred balances	1,653	(28,262)	920
Other	(609)	647	(333)
Total provision	$ (1,329)	$ (909)	$ (3,597)