Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Allowance for doubtful accounts receivable, current	$ 11	$ 8
Stockholders' equity:
Preferred stock, par value (in dollars per share)	$ 25	$ 25
Preferred stock, shares authorized (in shares)	10,000,000	10,000,000
Preferred stock, shares issued (in shares)	0	0
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares authorized (in shares)	2,400,000,000	2,400,000,000
Common stock, shares issued (in shares)	1,740,815,939	1,740,815,939
Treasury stock (in shares)	821,461,787	808,784,381