Leasing Activity Leasing Activity, Lease Costs (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Leases [Abstract]
Fixed lease expense - operating leases	$ 291	$ 588
Variable lease expense	80	153
Other	(9)	(17)
Total lease costs	$ 362	$ 724