Stockholders' Equity (Details) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Change in the number of shares of Class A common stock outstanding
Shares outstanding as of beginning of period	84,395	92,433
Shares issued as part of equity-based compensation plans and the Employee Stock Purchase Plan (ESPP), net of restricted stock units (RSUs) surrendered	905	1,119
Shares repurchased into treasury stock	(226)	(9,157)
Shares outstanding as of end of period	85,074	84,395