Accrued expenses and other current liabilities - Summary of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Accrued expenses	$ 43,440	$ 43,235
Derivative instruments	4,390	555
Client liabilities	5,107	8,982
Other current liabilities	6,945	8,594
Accrued expenses and other current liabilities	$ 59,882	$ 61,366