Lease Obligations - Lessee - Other Information (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Leases [Abstract]
Operating leases, weighted average remaining lease term	9 years 1 month 2 days		9 years 21 days
Finance leases, weighted average remaining lease term	14 years 5 months 1 day		12 years 11 months 4 days
Operating leases, weighted average discount rate	2.95%		3.05%
Finance leases, weighted average discount rate	2.58%		2.39%
Cash paid for amounts included in the measurement of lease liabilities:(a)
Operating cash flows from operating leases	$ 44	$ 47
Operating cash flows from finance leases	2	1
Financing cash flows from finance leases	9	2
Gains on sale-leaseback transactions	$ 2	$ 0