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Summary of Significant Accounting Policies - Additional Information (Details)
$ / shares in Units, $ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Dec. 09, 2020
Installment
shares
Sep. 30, 2021
USD ($)
Customer
$ / shares
shares
Sep. 30, 2020
Customer
Sep. 30, 2021
USD ($)
Customer
$ / shares
shares
Sep. 30, 2020
Customer
Dec. 31, 2020
USD ($)
Customer
$ / shares
Mar. 31, 2021
Jan. 14, 2021
Summary Of Significant Accounting Policies [Line Items]                
Description of significant off-balance sheet risk       The Company has no significant off‑balance sheet risk, such as foreign exchange contracts, option contracts, or other foreign hedging arrangements.        
ASU 2019-12                
Summary Of Significant Accounting Policies [Line Items]                
Change in accounting principle, accounting standards update, adopted   true   true        
Change in accounting principle, accounting standards update, adoption date   Jan. 01, 2021   Jan. 01, 2021        
Change in accounting principle, accounting standards update, immaterial effect   true   true        
Change in accounting principle, accounting standards update, transition option elected       ASU 2019-12        
2020 CO Plan                
Summary Of Significant Accounting Policies [Line Items]                
Requisite service period       3 years        
Number of shares authorized for issuances | shares   485,714   485,714        
2021 CO Plan                
Summary Of Significant Accounting Policies [Line Items]                
Requisite service period       3 years        
Number of shares authorized for issuances | shares   228,571   228,571        
Class A Common Stock                
Summary Of Significant Accounting Policies [Line Items]                
Common stock, par value | $ / shares   $ 0.001   $ 0.001   $ 0.001    
Car Offer                
Summary Of Significant Accounting Policies [Line Items]                
Business acquisition, percentage of interest acquired   51.00%   51.00%     51.00% 51.00%
Car Offer | Vesting Agreement                
Summary Of Significant Accounting Policies [Line Items]                
Business acquisition, percentage of interest acquired 51.00%              
Vesting number of shares | shares 432,592              
Number of Installment | Installment 3              
Car Offer | Restricted Stock Units                
Summary Of Significant Accounting Policies [Line Items]                
Requisite service period       4 years        
Car Offer | Stock Options                
Summary Of Significant Accounting Policies [Line Items]                
Requisite service period       4 years        
Vesting period       10 years        
Car Offer | Class A Common Stock                
Summary Of Significant Accounting Policies [Line Items]                
Common stock, par value | $ / shares   $ 0.001   $ 0.001        
Sales Revenue, Net | Concentration of Credit Risk                
Summary Of Significant Accounting Policies [Line Items]                
Number of major customers | Customer   0 0 0 0      
Net Accounts Receivable | Advertising Customers                
Summary Of Significant Accounting Policies [Line Items]                
Unbilled accounts receivable | $   $ 6,694   $ 6,694   $ 7,426    
Net Accounts Receivable | Concentration of Credit Risk | Customer One                
Summary Of Significant Accounting Policies [Line Items]                
Concentration of credit risk, percentage       30.00%   10.00%    
Number of major customers | Customer           1    
Net Accounts Receivable | Concentration of Credit Risk | Customer Two                
Summary Of Significant Accounting Policies [Line Items]                
Concentration of credit risk, percentage       15.00%        
Number of major customers | Customer       2