Capitalization (Details) (USD $)	6 Months Ended				0 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Direct Stock Purchase And Dividend Reinvestment Plan [Member]	Sep. 30, 2012 5.25% Notes Due in March 2013 [Member]	Feb. 15, 2013 3.75% Notes Due In March 1, 2023 [Member]
Debt Instrument [Line Items]
Common stock shares issued due to stock option exercises	411,625
Issue shares of common stock for the Direct Stock Purchase and Dividend Reinvestment Plan			68,306
Common stock issued	7,980
Shares tendered	296,306
Current portion of long-term debt				$ 250,000,000
Long-term debt, face value					500,000,000
Long-term debt, interest rate				5.25%	3.75%
Percentage of principal amount					101.00%
Net proceeds from issuance of long-term debt	$ 495,415,000	$ 496,085,000			$ 495,400,000