Long-term and Short-term Debt (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Long-term debt
Unamortized discount and other	$ 45.0	$ 45.0
Long-term debt	905.0	905.0
Line of Credit Facility, Maximum Borrowing Capacity	750.0	750.0
5.65% notes, payable in 2017 [Member]
Long-term debt
Debt instruments	250.0	250.0
Interest rate	5.65%
Maturity date	Dec. 01, 2017
6.70% debentures, payable in 2028 [Member]
Long-term debt
Debt instruments	250.0	250.0
Interest rate	6.70%
Maturity date	Jan. 15, 2028
6.25% debentures, payable in 2037 [Member]
Long-term debt
Debt instruments	250.0	250.0
Interest rate	6.25%
Maturity date	Dec. 01, 2037
5.20% debentures, payable in 2098 [Member]
Long-term debt
Debt instruments	$ 200.0	$ 200.0
Interest rate	5.20%
Maturity date	Jan. 15, 2098