Leasing Activity Leasing Activity, Lease Costs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Leases [Abstract]
Fixed lease expense – operating leases	$ 291	$ 297
Variable lease expense	66	73
Other	(14)	(8)
Total lease costs	$ 343	$ 362