CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
VIEs
Cash and cash equivalents	$ 170,385	$ 135,823	$ 161,122	$ 139,465
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	51,448,753	51,576,442
Common stock, shares outstanding	51,448,753	51,576,442
VIEs
VIEs
Cash and cash equivalents	$ 60,256	$ 7,045