Leases Lease Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Lease Costs [Abstract]
Operating lease cost	$ 286	$ 282
Variable lease cost	39	49
Sublease income	(99)	(89)	$ (72)
Net lease cost	$ 226	$ 242