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Stock-Based Compensation - Employee Stock Purchase Plans (Details)
 12 Months Ended
Dec. 31, 2023
USD ($)
StockPurchasePlan
$ / shares
shares
Dec. 31, 2022
USD ($)
$ / shares
shares
Dec. 31, 2021
USD ($)
$ / shares
shares
Dec. 31, 2023
GBP (£)
shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Number of employee stock purchase plans | StockPurchasePlan 3      
United States Stock Repurchase Program | Class A Non-Voting        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Shares authorized (in shares) 1,200,000     1,200,000
Percentage of elgible annual earnings 85.00%      
Maximum annual earnings withheld to purchase shares | $ $ 25,000      
Purchase price of stock, percent of market price (percent) 85.00%      
Shares issued (in shares) 149,170 120,727 155,293  
Purchase price of shares during period (in usd per share) | $ / shares $ 6.44 $ 6.63 $ 6.77  
Projected exercises in period (in shares) 130,000     130,000
Share-based compensation expense | $ $ 368,000 $ 314,000 $ 349,000  
U.K. Stock Repurchase Plan | Class A Non-Voting        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Shares authorized (in shares) 2,000,000     2,000,000
Purchase price of stock, percent of market price (percent) 85.00%      
Shares issued (in shares) 71,642 0 76,457  
Share-based compensation expense | $ $ 209,000 $ 155,000 $ 241,000  
Maximum monthly earnings withheld to purchase shares (in gbp) | £       £ 250
Estimated shares eligible for purchase (in shares) 214,000     214,000
U.K. Stock Repurchase Plan | Class A Non-Voting | Minimum        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Purchase price of shares during period (in usd per share) | $ / shares $ 5.17      
U.K. Stock Repurchase Plan | Class A Non-Voting | Maximum        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Purchase price of shares during period (in usd per share) | $ / shares $ 6.64      
International stock based compensation plan | Class A Non-Voting        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Shares authorized (in shares) 1,000,000     1,000,000
Shares issued (in shares) 5,026 3,355 4,080  
Purchase price of stock, percent of market price (percent)   85.00%    
International stock based compensation plan | Class A Non-Voting | Minimum        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Maximum annual earnings withheld to purchase shares | $ $ 21,250      
International stock based compensation plan | Class A Non-Voting | Maximum        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Maximum number of shares per employee 5,000