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Stock-Based Compensation - Employee Stock Purchase Plans (Details)
12 Months Ended
Dec. 31, 2019
USD ($)
stock_purchase_plan
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2019
GBP (£)
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of employee stock purchase plans | stock_purchase_plan 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) 131,100 143,769 101,708  
Purchase price of shares during period (in usd per share) | $ / shares $ 7.38 $ 7.32 $ 6.61  
Projected exercises in period (in shares) 102,000     102,000
Share-based compensation expense | $ $ 277,000 $ 321,000 $ 278,000  
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
Shares issued (in shares) 289,901 63,033 73,986  
Share-based compensation expense | $ $ 148,000 $ 140,000 $ 151,000  
Maximum monthly earnings withheld to purchase shares (in gbp) | £       £ 250
Purchase price of stock, percent of market price (percent) 85.00%      
Estimated shares eligible for purchase (in shares) 102,000     102,000
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 $ 6.32      
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
Maximum annual earnings withheld to purchase shares | $ $ 21,250      
Shares issued (in shares) 4,264 8,740 8,342  
Purchase price of stock, percent of market price (percent) 85.00%      
Maximum number of shares per employee 5,000