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Summary of significant accounting policies - Additional Information (Details) - USD ($)
 3 Months Ended 12 Months Ended
May 01, 2018
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Jan. 01, 2019
Accounting Policies and General Information [Line Items]              
Allowance for doubtful accounts     $ 9,329,000 $ 7,604,000 $ 7,709,000 $ 31,000  
Impairment of long-term investments     $ 794,000 19,831,000 7,794,000    
Cash flow projection period     5 years        
Discount rate     18.20%        
Terminal Growth Rate to extrapolate the cash flows beyond five-year period     2.00%        
Goodwill impairment losses     $ 0 0 0    
Sales rebates provided to the agencies and advertisers     0 0 394,000    
External advertising and market promotion expenses from continuing operations     $ 11,026,000 20,974,000 22,935,000    
Value added tax on revenues 17.00% 16.00% 13.00%        
Value added tax on sub-licensing revenues     6.00%        
Employee benefit costs     $ 7,949,000 $ 12,337,000 $ 12,501,000    
Statutory general reserve rate     10.00%        
Statutory general reserve rate of registered capital     50.00%        
Dividends declared     $ 0 $ 0 $ 0    
Income (Loss) from Equity Method Investments         $ (307,000)    
Right-of-use assets     $ 1,954,000 $ 8,747,000 11,819,000    
Lease liabilities     1,988,000 8,825,000      
Shenzhen Mojingou Information Service Co., Ltd [Member]              
Accounting Policies and General Information [Line Items]              
Income (Loss) from Equity Method Investments     0 0 317,000    
Long-term Investments [Member]              
Accounting Policies and General Information [Line Items]              
Impairment of long-term investments     794,000 19,830,000 $ 7,794,000    
Income (Loss) from Equity Method Investments     $ 214,000 $ 579,000      
Minimum              
Accounting Policies and General Information [Line Items]              
Term of time-based subscriptions     1 month        
Maximum              
Accounting Policies and General Information [Line Items]              
Term of time-based subscriptions     12 months        
Advertising contract term     3 months        
Sales Revenue, Net [Member] | Geographic Concentration Risk [Member] | China [Member] | Minimum              
Accounting Policies and General Information [Line Items]              
Percent of revenue       99.00% 99.00% 99.00%  
Accounting Standards Update 2016-02 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member] | Revision of Prior Period, Adjustment [Member]              
Accounting Policies and General Information [Line Items]              
Right-of-use assets             $ 11,800,000
Lease liabilities             $ 11,400,000
Land use right [Member]              
Accounting Policies and General Information [Line Items]              
Other intangible assets, amortization period     30 years