CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 107	$ 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)	113,741,001	112,907,209
Common stock, shares outstanding (in shares)	77,431,831	79,872,956
Treasury stock, shares (in shares)	36,309,170	33,034,253