Restricted Net Assets (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restricted net assets
Minimum required statutory reserve of annual after-tax profit (as a percent)	10.00%
Required statutory reserve as a percentage of registered capital (as a percent)	50.00%
Appropriation to statutory reserves	$ 0	$ 0	$ 0
China
Restricted net assets
Restricted net assets	$ 119,776,000	$ 109,633,000