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Commitments and Contingencies - Additional Information (Details)
£ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
Dec. 31, 2018
GBP (£)
Dec. 31, 2017
USD ($)
Dec. 31, 2017
GBP (£)
Dec. 31, 2016
USD ($)
Mar. 10, 2019
USD ($)
Dec. 31, 2018
GBP (£)
Dec. 31, 2017
GBP (£)
Loss Contingencies [Line Items]                
Capital expenditure commitments $ 2,500,000   $ 600,000   $ 3,600,000      
Long-term debt, gross 78,500,000              
Committed Banking Facilities                
Loss Contingencies [Line Items]                
Debt maximum borrowing capacity 150,000,000              
Amount drawn/utilized 0   1,300,000          
Other - Bank overdraft                
Loss Contingencies [Line Items]                
Long-term debt, gross 3,500,000   4,200,000          
Bonding Facility For Bank Guarantees                
Loss Contingencies [Line Items]                
Debt maximum borrowing capacity 3,800,000   4,100,000       £ 3.0  
Amount drawn/utilized 1,900,000 £ 1.5 1,400,000 £ 1.0        
Letter of Credit                
Loss Contingencies [Line Items]                
Debt maximum borrowing capacity 8,900,000   9,500,000       £ 7.0 £ 7.0
Amount drawn/utilized $ 0   $ 0          
Subsequent Event                
Loss Contingencies [Line Items]                
Payment committed related to aborted acquisition (up to)           $ 3,500,000