Stockholders' Equity (Details) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Change in the number of shares of Class A common stock outstanding
Shares outstanding as of beginning of period	85.2	84.4
Shares issued as part of equity-based compensation plans and the Employee Stock Purchase Plan (ESPP), net of restricted stock units surrendered	1.0	1.1
Common stock repurchases	(2.0)	(0.3)
Shares outstanding as of end of period	84.2	85.2