Consolidated Balance Sheets (Parenthetical) - USD ($) shares in Millions, $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Consolidated Balance Sheets
Accounts receivable, allowance for doubtful accounts	$ 33.5	$ 23.0
Class A Common Stock, par value (in dollars per share)	$ 0.001	$ 0.001
Class A Common Stock, shares authorized	1,000.0	1,000.0
Class A Common Stock, shares issued	299.2	305.7
Class A Common Stock, shares outstanding	298.5	305.7
Treasury stock, shares	0.7