Credit Arrangements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Nov. 30, 2012	May 31, 2012	Nov. 30, 2011
Debt Instrument [Line Items]
Net proceeds from termination of interest rate swaps		$ 53
Deferred gain on termination of interest rate swaps	44.0
Amortization of deferred gain from termination of interest rate fair value hedge	2.9
Interest costs capitalized	0.5		0.4
Interest paid	4.6		4.4
CMCZ [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity	77.6
Line of credit facility, amount outstanding	12.6
Short-term debt weighted average interest rate	5.80%
Revolving credit facility [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity	300.0
Revolving credit facility, maturity date	Dec. 27, 2016
Revolving credit facility, maximum borrowing capacity	400
Minimum interest coverage ratio	300.00%
Actual interest coverage ratio	612.00%
Maximum debt to capitalization ratio	60.00%
Actual debt to capitalization ratio	49.00%
Letter of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity	50.0
Stand by letters of credit outstanding amount	$ 30.7