Common Stock (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($) shares	Jul. 27, 2015 USD ($)	Apr. 30, 2013 USD ($)
Common Stock
Common Stock conversion ratio, at the option of the holder	1
Class A
Common Stock
Common Stock, votes per share (Number of votes)	1	1
Shares of Common Stock reserved for issuance under stock-based compensation plans	3,122,011
Options to purchase shares of Class A common stock | $			$ 100.0	$ 90.0
Number of shares repurchased	812,540	669,681
Cost of shares of Class A common stock repurchased | $	$ 44.6	$ 39.6
Class A | April 30, 2013
Common Stock
Number of shares repurchased	497,010	669,681
Cost of shares of Class A common stock repurchased | $	$ 27.3	$ 39.6
Class A | July 27, 2015
Common Stock
Number of shares repurchased	315,530
Cost of shares of Class A common stock repurchased | $	$ 17.3
Class B
Common Stock
Common Stock, votes per share (Number of votes)	10	10
Shares of Common Stock reserved for conversion	6,379,290