CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 103	$ 93
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	500,000,000	500,000,000
Common stock, shares issued (in shares)	113,778,857	112,907,209
Common stock, shares outstanding (in shares)	75,750,691	79,872,956
Treasury stock, shares (in shares)	38,028,166	33,034,253