Consolidated Balance Sheets (Parenthetical) - USD ($) shares in Millions, $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Consolidated Balance Sheets
Accounts receivable, allowance for doubtful accounts	$ 23.6	$ 25.6
Accumulated depreciation	$ 1,007.2	$ 900.9
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	308.3	308.0
Class A Common Stock, shares outstanding	308.3	308.0