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Employee Benefit Plans - Employer Contributions (Details) - 9 months ended Sep. 30, 2020
£ in Millions, $ in Millions
USD ($)
GBP (£)
GBP (£)
Other Postretirement Benefits Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, expected contributions in current fiscal year $ 1    
Employer contributions 1    
UNITED STATES | Pension Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, expected contributions in current fiscal year 13    
Employer contributions 9    
UNITED KINGDOM | Pension Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, expected contributions in current fiscal year | £     £ 43
Employer contributions $ 41 £ 32