Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Components of lease expense [Abstract]
Operating lease cost	$ 188
Finance lease cost:
Amortization of right-of-use assets	28
Interest on lease liabilities	3
Total finance lease costs	31
Variable lease cost	50
Total lease costs	269
Supplemental cash flow information related to leases [Abstract]
Operating cash flows from operating leases	173
Operating cash flows from finance leases	3
Financing cash flows from financing leases	25
Right-of-use asset obtained in exchange for operating lease obligations	89
Right-of-use asset obtained in exchange for financing lease obligations	68
Operating and finance lease ROU assets and liabilities [Abstract]
Operating lease right-of-use assets	536
Accrued expenses and other current liabilities	166
Other non-current liabilities	465
Total operating lease liabilities	$ 631
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Other assets noncurrent
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Accrued Expenses And Other Liabilities
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Other non-current liabilities
Finance leases: Property and equipment, gross	$ 110
Finance leases: Property and equipment, accumulated depreciation	(23)
Finance leases: Property and equipment, net	87
Finance leases: Short-term debt	27	$ 117
Finance leases: Long-term debt	61	32
Total finance lease liabilities	$ 88
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Property and equipment
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	Short Term Borrowings And Long Term Debt Current
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Long-term debt, carrying value
Weighted average remaining lease term, operating leases	5 years
Weighted average remaining lease term, finance leases	5 years
Weighted average discount rate, operating leases	3.89%
Weighted average discount rate, finance leases	3.77%
Net rental expense for operating leases		$ 162	$ 162
Accounting Standards Update 2016-02 [Member]
Operating and finance lease ROU assets and liabilities [Abstract]
Operating lease right-of-use assets				$ 615
Total operating lease liabilities				$ 630
Accrued Expenses And Other Liabilities [Member]
Operating and finance lease ROU assets and liabilities [Abstract]
Total operating lease liabilities	$ 166
Other Liabilities Noncurrent [Member]
Operating and finance lease ROU assets and liabilities [Abstract]
Total operating lease liabilities	$ 465
Maximum [Member]
Leases [Line Items]
Term length for operating leases	23 years