Common Stock (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015	Jul. 27, 2015 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 | shares	2,673,034
Options to purchase shares of Class A common stock | $				$ 100.0
Stock Repurchase Program, Remaining Authorized Repurchase Amount | $	$ 37.8
Number of shares repurchased | shares	277,886	501,229
Cost of shares of Class A common stock repurchased | $	$ 18.2	$ 26.8
Class A | July 27, 2015
Common Stock
Number of shares repurchased | shares	277,886	501,229
Cost of shares of Class A common stock repurchased | $	$ 18.2	$ 26.8
Class B
Common Stock
Common Stock, votes per share (Number of votes)	10	10	10
Shares of Common Stock reserved for conversion | shares	6,379,290