Cash, cash equivalents and restricted cash (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash and cash equivalents	$ 140,957	$ 133,407	$ 69,429
Short-term and long-term restricted cash	7,514		6,792
Total cash, cash equivalents and long-term restricted cash	148,471		76,221	$ 140,880	$ 77,655
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		$ 6,800
Revolving credit facility | Beverly Property Owner LLC
Letter of credit associated with security deposit for leaseback transaction	$ 5,900