Stockholders' Equity - Changes in Class A Common Stock (Details) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Change in the number of shares of Class A common stock outstanding [Roll Forward]
Shares outstanding as of end of period	85.1	85.2
Class A Common Stock
Change in the number of shares of Class A common stock outstanding [Roll Forward]
Shares outstanding as of beginning of period	85.2	84.4
Shares issued as part of equity-based compensation plans and the ESPP, net of shares surrendered	1.9	1.1
Shares repurchased into treasury stock	(2.0)	(0.3)
Shares outstanding as of end of period	85.1	85.2