Leases - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Property
Cash payments for lease liabilities within operating activities:
Operating leases	$ 639	$ 740	$ 733
Non-cash activities - increase (decrease) in ROU assets in exchange for lease liabilities:
Operating leases	484	666	531
Buses and support vehicles
Cash payments for lease liabilities within operating activities:
Operating leases	162	196	248
Vehicle finance leases	4	3	4
Cash payments for lease liabilities within financing activities:
Vehicle finance leases	193	294	266
Non-cash activities - increase (decrease) in ROU assets in exchange for lease liabilities:
Operating leases	115	111	262
Vehicle finance leases	$ 223	$ 257	$ 304