Leases - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Property leases
Cash payments for lease liabilities within operating activities:
Operating leases	$ 947	$ 838	$ 743
Non-cash activities - increase in ROU assets in exchange for lease liabilities:
Operating leases	1,404	1,079	812
Buses and support vehicles
Cash payments for lease liabilities within operating activities:
Operating leases	151	167	137
Vehicle finance leases	7	6	3
Cash payments for lease liabilities within financing activities:
Vehicle finance leases	105	105	181
Non-cash activities - increase in ROU assets in exchange for lease liabilities:
Operating leases	112	191	161
Vehicle finance leases	$ 102	$ 118	$ 153