Stockholders' Equity (Details Textual) (USD $)	1 Months Ended		5 Months Ended	6 Months Ended	11 Months Ended	17 Months Ended
	Apr. 30, 2012 Class A and Class B [Member]	Apr. 30, 2011 Class A and Class B [Member]	Aug. 31, 2012 Class A Common Stock [Member]	Aug. 31, 2012 Class A Common Stock [Member]	Feb. 29, 2012 Class A Common Stock [Member]	Aug. 31, 2012 Class A Common Stock [Member]
Class of Stock [Line Items]
Maximum authorized repurchase of common stock	$ 1,000,000,000.0	$ 500,000,000
Common stock repurchased, shares			14,023,985	3,970,481	21,234,266	25,204,747
Common stock repurchased			$ 296,700,000	$ 86,300,000	$ 413,700,000	$ 500,000,000
Average cost of common stock repurchased			$ 21.15	$ 21.74	$ 19.48	$ 19.84