Stockholders' Equity (Details) - shares shares in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Change in the number of shares of Class A common stock outstanding
Shares outstanding as of beginning of period	85.1	85.2
Shares issued as part of equity-based compensation plans and the ESPP, net of RSUs surrendered	1.1	1.9
Common stock repurchases	0.0	(2.0)
Shares outstanding as of end of period	86.2	85.1