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Stock-Based Compensation - Employee Stock Purchase Plans (Details)
12 Months Ended
Dec. 31, 2021
USD ($)
StockPurchasePlan
$ / shares
shares
Dec. 31, 2020
USD ($)
$ / shares
shares
Dec. 31, 2019
USD ($)
$ / shares
shares
Dec. 31, 2021
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
Maximum annual earnings withheld to purchase shares | $ $ 25,000      
Purchase price of stock, percent of market price (percent) 85.00%      
Shares issued (in shares) 155,293 114,408 131,100  
Purchase price of shares during period (in usd per share) | $ / shares $ 6.77 $ 6.71 $ 7.38  
Projected exercises in period (in shares) 128,000     128,000
Share-based compensation expense | $ $ 349,000 $ 343,000 $ 277,000  
U.K. Stock Repurchase Plan | Class A Non-Voting        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Shares authorized (in shares) 1,200,000     1,200,000
Purchase price of stock, percent of market price (percent) 85.00%      
Shares issued (in shares) 76,457 2,061 289,901  
Share-based compensation expense | $ $ 241,000 $ 163,000 $ 148,000  
Maximum monthly earnings withheld to purchase shares (in gbp) | £       £ 250
Estimated shares eligible for purchase (in shares) 196,000     196,000
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 $ 6.64      
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 $ 8.47      
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) 4,080 4,051 4,264  
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