Lease Obligations - Lessee - Other Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating leases, weighted average remaining lease term	9 years 21 days	9 years 5 months 23 days
Finance leases, weighted average remaining lease term	12 years 11 months 4 days	14 years 2 months 1 day
Operating leases, weighted average discount rate	3.05%	3.19%
Finance leases, weighted average discount rate	2.39%	4.30%
Cash paid for amounts included in the measurement of lease liabilities:(a)
Operating cash flows from operating leases	$ 91	$ 97
Operating cash flows from finance leases	3	1
Financing cash flows from finance leases	11	5
Gains on sale and leaseback transactions	$ 3	$ 5