Accrued expenses and other current liabilities - Summary of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Accrued expenses	$ 43,521	$ 43,235
Derivative instruments	7,870	555
Client liabilities	6,831	8,982
Other current liabilities	8,862	8,594
Accrued expenses and other current liabilities	$ 67,084	$ 61,366