Leases Capital Leases, Balance Sheet, Assets by Major Class, Net (Details) - USD ($) $ in Millions		Jun. 30, 2019	Dec. 31, 2018
Capital Leased Assets [Line Items]
2019			$ 111
2020			109
2021			105
2022			100
2023			97
Thereafter			335
Total minimum lease payments			857
Less amount representing interest			126
Present value of minimum lease payments (a)	[1]		731
Less current portion			(85)
Long-term portion			646
Lease Incentive Obligation Excluded			114
Subleases
2019			(92)
2020			(78)
2021			(41)
2022			(17)
2023			(7)
Thereafter			(1)
Total minimum lease payments			(236)
Operating Leased Assets [Line Items]
2019		$ 48	348
2020		78	357
2021		41	244
2022		17	172
2023		$ 7	146
Thereafter			474
Total minimum lease payments			1,741
2019			256
2020			279
2021			203
2022			155
2023			139
Thereafter			473
Total minimum lease payments			$ 1,505

[1] Excludes lease incentive obligation of $114 million.