CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Accounts receivable, allowance for doubtful accounts	$ 3	$ 2
Class A Common Stock
Common Stock, par value (USD per share)	$ 0.01	$ 0.01
Common Stock, shares authorized (shares)	500,000,000	500,000,000
Common Stock, shares issued (shares)	67,063,137	68,897,723
Common Stock, shares outstanding (shares)	67,063,137	68,897,723
Class B Common Stock
Common Stock, par value (USD per share)	$ 0.01	$ 0.01
Common Stock, shares authorized (shares)	140,000,000	140,000,000
Common Stock, shares issued (shares)	25,027,682	25,670,684
Common Stock, shares outstanding (shares)	25,027,682	25,670,684