Treasury Stock (Details) - Schedule of repurchase of common stock (USD $) In Millions, unless otherwise specified	12 Months Ended								66 Months Ended
	May 31, 2013	Sep. 30, 2010		Dec. 31, 2009	Feb. 28, 2009	Nov. 30, 2008	May 31, 2008	Dec. 31, 2007	May 31, 2013
Schedule of repurchase of common stock [Abstract]
Authorization Amount		$ 44.0	[1]	$ 20.0	$ 5.0	$ 10.0	$ 20.0	$ 20.0	$ 119.0
Less repurchases made from December 2007 through May 2013									(99.4)
Remaining Board authorization at May 31, 2013	$ 19.6

[1] Represents the remainder of a $200.0 authorization after giving effect to the purchase of 5,199,699 shares at $30.00 per share pursuant to a large share repurchase in the form of a modified Dutch auction tender offer that was completed by the Company on November 3, 2010 for a total cost of $156.0, excluding related fees and expenses.