Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Allowance for doubtful accounts	$ 17	$ 16
Property and equipment, accumulated depreciation	198	177
Intangible assets, accumulated amortization	$ 120	$ 112
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	136,396,872	135,617,263
Common stock, shares outstanding	126,669,732	126,142,773
Treasury stock, shares	9,727,140	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