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Revenue (Tables)
6 Months Ended
Jun. 30, 2021
Revenue from Contract with Customer [Abstract]  
Changes in Deferred Revenue The following table presents changes in the deferred revenue balance as of June 30:
20212020
Balance as of January 1$101,542 $121,339 
Deferral of revenue131,895 103,542 
Recognition of deferred revenue(105,590)(121,273)
Balance as of June 30$127,847 $103,608