Long-Term Debt and Financing Obligations Long term debt - Pollution Control Bonds (Details) (USD $) In Millions, unless otherwise specified	Aug. 28, 2012 1.875% 2012 Series A Refunding Bonds, due 2032 [Member]	Aug. 27, 2012 4.80% 2005 Series A Refunding Bonds, due 2040 [Member]	Aug. 28, 2012 4.50% 2012 Series A Refunding Bonds, due 2042 [Member]	Aug. 01, 2012 4.00% 2002 Series A Refunding Bonds, due 2032 [Member]	Sep. 30, 2012 Pollution Control Bonds [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 33.3	$ 59.2	$ 59.2	$ 33.3
Long-term Debt					$ 193.1
Debt Instrument, Interest Rate, Stated Percentage	1.875%	4.80%	4.50%	4.00%
Redemption Price of Notes Equal to Principal Amount, Percentage	100.00%
Debt Instrument Effective Interest Rate	2.25%		4.62%