Debt (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended				3 Months Ended
	Dec. 05, 2012	Aug. 31, 2013	Aug. 31, 2013 Unsecured Debt [Member]	Aug. 31, 2013 Secured Debt [Member]	Aug. 31, 2013 Federal Funds Rate [Member]	Aug. 31, 2013 Eurodollar Rate [Member]	Aug. 31, 2013 LIBOR Rate [Member]	Aug. 31, 2013 Standby Letter Of Credit [Member]	Aug. 31, 2013 Loan Agreement [Member]	Aug. 31, 2013 Secured Debt [Member]	May 31, 2013 Secured Debt [Member]	Aug. 31, 2012 Secured Debt [Member]
Debt (Details) [Line Items]
Line of Credit Facility, Current Borrowing Capacity		$ 425.0
Line of Credit Facility, Expiration Date	Dec. 05, 2017
Line of Credit Facility, Interest Rate During Period					0.50%
Line of Credit Facility, Interest Rate Description						Eurodollar Rate for a one month interest period plus 1% plus, in each case, an applicable spread ranging from 0.18% to 0.60%	London interbank offered rate (LIBOR) plus an applicable spread ranging from 1.18% to 1.60%
Line Of Credit Facility Base Rate Advances		0.18%
Line Of Credit Facility Eurodollar Rate Advances		1.18%
Line of Credit Facility, Fee Percentage Range Minimum		0.20%
Line of Credit Facility Fee Percentage Range Maximum		0.40%
Line Of Credit Facility Fee Percentage		0.20%
Line of Credit Facility, Amount Outstanding		15.0
Standby Letters of Credit								6.6	1.4
Credit Lines Available			13.9	30.0
Credit Lines Borrowed Amount			$ 5.9							$ 8.3	$ 2.0	$ 0.6