Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Allowance for doubtful accounts	$ 22	$ 21
Intangible assets, accumulated amortization	$ 155	$ 140
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	138,525,158	137,158,010
Common stock, shares outstanding	126,468,470	125,101,322
Treasury stock, shares	12,056,688	12,056,688
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