CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
VIEs
Cash and cash equivalents	$ 139,404	$ 170,385
Stockholders' equity
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized	90,000,000	90,000,000
Common stock, shares issued	51,530,572	51,448,753
Common stock, shares outstanding	51,530,572	51,448,753
VIEs
VIEs
Cash and cash equivalents	$ 38,750	$ 60,256