Cash, cash equivalents and restricted cash (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash and cash equivalents	$ 133,407	$ 70,791
Short-term and long-term restricted cash	7,473	6,864
Total cash, cash equivalents and restricted cash	140,880	$ 77,655	$ 85,825	$ 31,578
Letter of credit related to workers' compensation insurance	800
Letter of credit associated with a bank guarantee	700
Deposit related to customs activity	100
Revolving credit facility
Restricted cash which relates to support of outstanding letters of credit	7,500
Revolving credit facility | Sale leaseback obligation
Letter of credit associated with security deposit for leaseback transaction	$ 5,900