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Debt - Debt (Details)
£ in Millions, $ in Millions
9 Months Ended
Sep. 30, 2020
USD ($)
Sep. 30, 2020
GBP (£)
Mar. 30, 2020
USD ($)
Debt Instruments [Line Items]      
Line of Credit Facility, Covenant Compliance Navigators Group was in compliance with all financial covenants.    
Club Facility [Member]      
Debt Instruments [Line Items]      
Letters of Credit Outstanding, Amount $ 165 £ 60  
Bilateral Facility [Member]      
Debt Instruments [Line Items]      
Letters of Credit Outstanding, Amount | £   £ 18  
Debt Instrument, Unused Borrowing Capacity, Amount $ 2    
Senior Notes | Senior Note Six Point Zero Percent Due in Two Thousand Nineteen [Member]      
Debt Instruments [Line Items]      
Repayment of debt     $ 500