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Significant Accounting Policies - Concentrations and Risks (Details)
12 Months Ended
Dec. 31, 2020
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customer
Dec. 31, 2019
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customer
Dec. 31, 2018
Dec. 31, 2017
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Total number of online advertising and promotional service providers 41 25   32
Number of online service providers that accounted for 10% or more of the Group's online advertising and promotional service 2 2   2
Total percentage of the Group's online advertising and promotional service expenses that were paid to these service providers who accounted for 10% or more of the Group's online advertising and promotional service expenses. 43.00% 55.00%   43.00%
Credit Concentration Risk [Member] | Accounts Receivable [Member]        
Number of customers | customer 0 0    
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Customer A [Member]        
Concentration risk (as a percent) 10.00% 10.00% 10.00%