Debt (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended						1 Months Ended	12 Months Ended							12 Months Ended
	Dec. 05, 2012	May 31, 2008	May 31, 2013	May 31, 2012	May 31, 2003	May 31, 2013 Line of Credit [Member]	May 31, 2012 Line of Credit [Member]	Dec. 05, 2012 Revolving Loan [Member]	May 31, 2008 Revolving Loan [Member]	May 31, 2013 Federal Funds Rate [Member]	May 31, 2013 Eurodollar Rate [Member]	May 31, 2013 LIBOR Rate [Member]	May 31, 2003 Notes Payable, Other Payables [Member]	May 31, 2013 Unsecured Debt [Member]	May 31, 2013 Secured Debt [Member]	May 31, 2008 Term Loan [Member]	May 31, 2013 Standby Letter Of Credit [Member]	May 31, 2013 Loan Agreement [Member]
Debt (Details) [Line Items]
Credit Facility Sanctioned During Period		$ 525.0						$ 425.0	$ 325.0							$ 200.0
Additional Credit Facility Sanctioned									150.0
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%
Standby Letters of Credit																	6.6	1.4
Credit Lines Available														14.8	27.0
Debt, Long-term and Short-term, Combined Amount			2.0	159.3		2.0	6.5
Short-term Debt, Weighted Average Interest Rate						9.00%	5.30%
Notes Payable, Noncurrent, Value					$ 175.0
Debt Instrument, Interest Rate, Stated Percentage			5.00%	5.00%									5.00%