Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Allowance for doubtful accounts	$ 21	$ 16
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	100,000,000	100,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	1,600,000,000	1,600,000,000
Common stock, shares issued	137,158,010	135,617,263
Common stock, shares outstanding	125,101,322	126,142,773
Treasury stock, shares	12,056,688	9,474,490
Class B Common Stock
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	400,000,000	400,000,000
Common stock, shares issued	12,799,999	12,799,999
Common stock, shares outstanding	12,799,999	12,799,999