Cash, cash equivalents and restricted cash (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash and cash equivalents	$ 177,993	$ 133,407
Short-term and long-term restricted cash	6,909	7,473
Total cash, cash equivalents and long-term restricted cash	184,902	$ 140,880	$ 77,655	$ 85,825
Letter of credit related to workers' compensation insurance	800
Letter of credit associated with a bank guarantee	100
Deposit related to customs activity	100
Revolving credit facility
Restricted cash which relates to support of outstanding letters of credit	6,900
Revolving credit facility | Sale leaseback obligation
Letter of credit associated with security deposit for leaseback transaction	$ 5,900