Accrued expenses and other current liabilities - Summary of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounts Payable and Accrued Liabilities, Current [Abstract]
Accrued expenses	$ 43,235	$ 30,690
Derivative instruments	555	1,430
Client liability account	8,982	4,005
Others	8,594	7,139
Accrued expenses and other current liabilities	$ 61,366	$ 43,264