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Intangible Assets, Net (Tables)
3 Months Ended
Mar. 31, 2021
Goodwill and Intangible Assets Disclosure [Abstract]  
Schedule of intangible assets
   December 31,
   March 31, 
   2020   2021 
Computer software at cost  $32,686   $32,567 
Accumulated amortization   (20,879)   (21,483)
   $11,807   $11,084