Leases - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Property
Cash payments for lease liabilities within operating activities:
Operating leases	$ 743	$ 639	$ 740
Non-cash activities - increase in ROU assets in exchange for lease liabilities:
Operating leases	812	484	666
Buses and support vehicles
Cash payments for lease liabilities within operating activities:
Operating leases	137	162	196
Vehicle finance leases	3	4	3
Cash payments for lease liabilities within financing activities:
Vehicle finance leases	181	193	294
Non-cash activities - increase in ROU assets in exchange for lease liabilities:
Operating leases	161	115	111
Vehicle finance leases	$ 153	$ 223	$ 257