Commitments and Contingencies - Leases (Details) £ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2003 GBP (£)
Primary Capital Leases
Capital lease obligation	$ 92
Minimum lease payments under capital leases
2019	18
2020	16
2021	14
2022	12
2023	12
After 2023	32
Total	104
Less: Amounts representing interest	12
Less: Current obligations under capital leases	17
Long-term obligations under capital leases	75	$ 60
Minimum lease payments under operating leases
2019	283
2020	208
2021	153
2022	122
2023	92
After 2023	253
Total	1,111
Capital and Operating Leases:
Rental expense under operating leases	$ 393	343	$ 318
Capital and operating leases with non-cancelable terms, low end of range	1 year
Building in United Kingdom
Primary Capital Leases
Capital lease term (in years)				22 years
Capital lease obligation	$ 43			£ 34
Capital lease obligations in aggregate
Primary Capital Leases
Capital lease term (in years)	15 years
Capital lease obligation	$ 13	$ 13	$ 12