CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Accounts receivable, allowance	$ 32	$ 31
Treasury stock - Common stock, Shares	93,380,000	89,528,000
Common Stock
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, Authorized shares	1,600,000,000	1,600,000,000
Common stock, Shares issued	230,159,000	228,467,000
Common stock, Shares outstanding	136,779,000	138,939,000
Class B Common Stock
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, Authorized shares	400,000,000	400,000,000
Common stock, Shares issued	12,800,000	12,800,000
Common stock, Shares outstanding	12,800,000	12,800,000