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Leases - Disclosures related to periods prior to adoption of new lease standard (Details)
£ in Millions, $ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2003
GBP (£)
Capital and Operating Leases      
Rental expense under operating leases $ 393 $ 343  
Accumulated depreciation for capital leases 54    
Capital lease asset and obligation 92    
Building in United Kingdom      
Capital and Operating Leases      
Capital lease asset and obligation 43   £ 34
Capital lease term (in years)     22 years
Capital lease obligations in aggregate      
Capital and Operating Leases      
Capital lease asset and obligation $ 13 $ 13  
Capital lease term (in years) 15 years