Common Stock (Details) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended	3 Months Ended	12 Months Ended		0 Months Ended
	May 16, 2012	Sep. 30, 2011	Dec. 31, 2012	Aug. 02, 2011	May 16, 2012 Class A Common Stock	Dec. 31, 2012 Class A Common Stock	Dec. 31, 2011 Class A Common Stock	Aug. 02, 2011 Class A Common Stock	Dec. 31, 2012 Class B Common Stock	Dec. 31, 2011 Class B Common Stock
Common Stock
Common Stock, votes per share						1	1		10	10
Common Stock conversion ratio, at the option of the holder			1
Shares of Common Stock reserved for issuance under stock-based compensation plans						2,732,358
Shares of Common Stock reserved for conversion									6,588,680
Number of shares authorized to be repurchased	2,000,000			1,000,000	2,000,000			1,000,000
Number of shares repurchased	2,000,000	1,000,000			2,000,000
Cost of shares repurchased	$ 65.8	$ 27.2			$ 65.8