Leases - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Property leases
Cash payments for lease liabilities within operating activities:
Operating leases	$ 838	$ 743	$ 639
Non-cash activities - increase in ROU assets in exchange for lease liabilities:
Operating leases	1,079	812	484
Buses and support vehicles
Cash payments for lease liabilities within operating activities:
Operating leases	167	137	162
Vehicle finance leases	6	3	4
Cash payments for lease liabilities within financing activities:
Vehicle finance leases	105	181	193
Non-cash activities - increase in ROU assets in exchange for lease liabilities:
Operating leases	191	161	115
Vehicle finance leases	$ 118	$ 153	$ 223