Book overdrafts and cash pooling - Additional Information (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 120,389	$ 254,912	$ 257,226	$ 182,434
Outstanding A/P check
Cash and Cash Equivalents [Line Items]
Outstanding A/P checks	13,205	17,343
Outstanding payroll checks
Cash and Cash Equivalents [Line Items]
Outstanding A/P checks	114	259
Notional Pooling Arrangement
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 2,562	$ 3,328