Cash, cash equivalents and restricted cash (Details) $ in Thousands	Jun. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Cash and cash equivalents	$ 147,297	$ 133,407	$ 108,558
Short-term and long-term restricted cash	7,645		6,793
Total cash, cash equivalents and long-term restricted cash	154,942	$ 140,880	115,351	$ 77,655
Letter of credit related to workers' compensation insurance	$ 800
Number of bank guaranties | item	2
Letter of credit associated with bank guarantee one	$ 700
Letter of credit associated with bank guarantee two	100
Deposit related to customs activity	100
Revolving credit facility
Restricted cash which relates to support of outstanding letters of credit	7,600		$ 6,800
Revolving credit facility | Beverly Property Owner LLC
Letter of credit associated with security deposit for leaseback transaction	$ 5,900