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, 2016 USD ($)	Dec. 31, 2003 GBP (£)
Capital and Operating Leases
Rental expense under operating leases	$ 393	$ 343	$ 318
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	$ 12
Capital lease term (in years)	15 years