Interim financial statements (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Statutory reserve funds, short-term investments	$ 479,155	$ 485,409
Statutory reserve funds, cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Statutory reserve funds, cash and cash equivalents	23,300	13,900
Statutory reserve funds, short-term investments
Restricted Cash and Cash Equivalents Items [Line Items]
Statutory reserve funds, short-term investments	$ 479,200	$ 485,400