Leases (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Components of lease expense [Abstract]
Operating lease cost	$ 47	$ 94
Finance lease cost:
Amortization of right-of-use assets	6	14
Interest on lease liabilities	1	2
Total finance lease costs	7	16
Variable lease cost	11	23
Total lease costs	65	133
Supplemental cash flow information related to leases [Abstract]
Operating cash flows from operating leases		84
Operating cash flows from finance leases		2
Financing cash flows from financing leases		12
Right-of-use asset obtained in exchange for operating lease obligations		49
Right-of-use asset obtained in exchange for financing lease obligations		53
Operating and finance lease ROU assets and liabilities [Abstract]
Operating lease right-of-use assets	587	587
Accrued expenses and other current liabilities	163	163
Other non-current liabilities	512	512
Total operating lease liabilities	$ 675	$ 675
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Other assets noncurrent	Other assets noncurrent
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Accrued Expenses And Other Liabilities	Accrued Expenses And Other Liabilities
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Other non-current liabilities	Other non-current liabilities
Finance leases: Property and equipment, net	$ 93	$ 93
Finance leases: Short-term debt	22	22	$ 117
Finance leases: Long-term debt	70	70	$ 32
Total finance lease liabilities	$ 92	$ 92
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Property and equipment	Property and equipment
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	Short Term Borrowings And Long Term Debt Current	Short Term Borrowings And Long Term Debt Current
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Long-term debt, carrying value	Long-term debt, carrying value
Weighted average remaining lease term, operating leases	5 years	5 years
Weighted average remaining lease term, finance leases	6 years	6 years
Weighted average discount rate, operating leases	3.89%	3.89%
Weighted average discount rate, finance leases	3.87%	3.87%
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	$ (12)	$ (12)
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