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Debt - UK Bilateral Revolving Credit Facility (Details) - UK Bilateral Revolving Credit Facility - GBP (£)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Debt Instrument [Line Items]    
Effective interest rate (as a percent) 2.30% 3.10%
Revolving Credit Facility    
Debt Instrument [Line Items]    
Maximum borrowing capacity £ 140,000,000  
Optional additional commitments £ 125,000,000  
Debt instrument, basis spread on variable rate 2.25%