Leases - Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Supplemental Balance Sheet Information Related to Leases [Line Items]
Operating lease ROU assets	$ 2,368	$ 2,560
Short-term operating lease liabilities	496	514
Long-term operating lease liabilities	$ 1,910	$ 2,078
Weighted average discount rate	1.25%	1.16%
Operating Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration]	Accounts Payable and Accrued Liabilities, Current	Accounts Payable and Accrued Liabilities, Current
Finance Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration]	Vehicles, net	Vehicles, net
Finance Lease, Liability, Statement of Financial Position [Extensible Enumeration]	Total	Total
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	Operating lease ROU assets	Operating lease ROU assets
Operating Lease, Liability, Statement of Financial Position [Extensible Enumeration]	Operating lease ROU assets	Operating lease ROU assets
Property
Supplemental Balance Sheet Information Related to Leases [Line Items]
Operating lease ROU assets	$ 2,368	$ 2,560
Short-term operating lease liabilities	496	514
Long-term operating lease liabilities	1,910	2,078
Operating lease liabilities	$ 2,406	$ 2,592
Weighted average remaining lease term	8 years 9 months 18 days	8 years 4 months 24 days
Weighted average discount rate	3.84%	3.86%
Buses and support vehicles
Supplemental Balance Sheet Information Related to Leases [Line Items]
Operating lease ROU assets	$ 63	$ 113
Short-term operating lease liabilities	50	82
Long-term operating lease liabilities	14	31
Operating lease liabilities	$ 64	$ 113
Weighted average remaining lease term	1 year 3 months 18 days	1 year 6 months
Weighted average discount rate	2.66%	2.90%
Finance lease ROU assets, gross	$ 353	$ 304
Accumulated amortization	(72)	(42)
Finance Lease ROU assets, net	281	262
Short-term vehicle finance lease liabilities	126	86
Long-term vehicle finance lease liabilities	116	140
Total	$ 242	$ 226
Weighted average remaining lease term	2 years 2 months 12 days	1 year 9 months 18 days
Weighted average discount rate	1.97%	1.99%