Leases (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Components of lease expense [Abstract]
Operating lease cost	$ 45	$ 139
Finance lease cost:
Amortization of right-of-use assets	6	20
Interest on lease liabilities	0	2
Total finance lease costs	6	22
Variable lease cost	13	36
Total lease costs	64	197
Supplemental cash flow information related to leases [Abstract]
Operating cash flows from operating leases		126
Operating cash flows from finance leases		2
Financing cash flows from financing leases		17
Right-of-use asset obtained in exchange for operating lease obligations		70
Right-of-use asset obtained in exchange for financing lease obligations		60
Operating and finance lease ROU assets and liabilities [Abstract]
Operating lease right-of-use assets	565	565
Accrued expenses and other current liabilities	162	162
Other non-current liabilities	495	495
Total operating lease liabilities	657	657
Finance leases: Property and equipment, net	88	88
Finance leases: Short-term debt	23	23	$ 117
Finance leases: Long-term debt	64	64	$ 32
Total finance lease liabilities	$ 87	$ 87
Weighted average remaining lease term, operating leases	5 years	5 years
Weighted average remaining lease term, finance leases	5 years	5 years
Weighted average discount rate, operating leases	3.92%	3.92%
Weighted average discount rate, finance leases	3.84%	3.84%
Property and equipment, gross [Member]
Operating and finance lease ROU assets and liabilities [Abstract]
Finance leases: Property and equipment, net	$ 105	$ 105
Accumulated depreciation [Member]
Operating and finance lease ROU assets and liabilities [Abstract]
Finance leases: Property and equipment, net	$ (17)	$ (17)
Minimum [Member]
Leases [Line Items]
Term length for operating leases	1 month	1 month
Maximum [Member]
Leases [Line Items]
Term length for operating leases	18 years	18 years