Other Expenses (Restructuring Charges) (Details) - Other expenses - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Lease and Asset Impairment
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 0	$ 43	$ 12
Unit Cost Initiative | Severance
Restructuring Cost and Reserve [Line Items]
Balance at January 1,	57	23	22
Restructuring charges	0	108	63
Cash payments	(51)	(74)	(62)
Balance at December 31,	6	57	23
Total restructuring charges incurred since inception of initiative	$ 244	$ 244	$ 136