Leases (Commitments Relating to Noncancellable Lease Obligations) (Details) - USD ($) $ in Millions		Mar. 31, 2019	Dec. 31, 2018
Operating Leases
2019 (remaining nine months)		$ 157
2020		170
2021		123
2022		100
2023		79
Thereafter		231
Total minimum future payments		860
Less: Imputed interest		(222)
Total liabilities		638
Less: current portion		(143)
Long-term liabilities		495
Finance Leases
2019 (remaining nine months)		12
2020		12
2021		11
2022		10
2023		16
Thereafter		164
Total minimum future payments		225
Less: Imputed interest		(99)
Total liabilities		126
Less: Current portion		(8)
Long-term liabilities		118
Financing Obligations
2019 (remaining nine months)		7
2020		7
2021		7
2022		7
2023		7
Thereafter		119
Total minimum future payments		154
Less: Imputed interest		(52)
Total financing obligations		102
Less: Current portion		(3)
Long-term liabilities		$ 99
Lease Accounting Prior to Adoption of ASC 842 [Member]
Capital Leases
2019			$ 12
2020			10
2021			8
2022			7
2023			14
Thereafter			121
Total minimum future payments			172
Less: Imputed interest			(80)
Total capital lease obligations			92
Less: Current portion			(8)
Long-term capital lease obligations			84
Operating Leases
2019	[1]		188
2020	[1]		157
2021	[1]		121
2022	[1]		98
2023	[1]		79
Thereafter	[1]		234
Total minimum future payments	[1]		877
Financing Obligations
2019			12
2020			9
2021			10
2022			10
2023			10
Thereafter			106
Total minimum future payments			157
Less: Imputed interest			(18)
Total financing obligations			139
Less: Current portion			(5)
Long-term liabilities			$ 134

[1] Minimum lease payments have not been reduced by minimum sublease rentals due in the future, which are considered immaterial.