Debt - Debt Instruments Additional Information (Details) (USD $)	3 Months Ended
	Jun. 30, 2013	Sep. 30, 2012
Long Term Debt Other Disclosures [Abstract]
Authorized Commercial Paper	$ 750,000,000
Commercial Paper	142,000,000	310,900,000
Debt and Equity Securities Authorized for Issuance	1,750,000,000
Debt And Equity Securities Available For Issuance	1,750,000,000
Regulated Operations Line of Credit Facilities, Covenant Terms	We are required by the financial covenants in each of these facilities to maintain, at the end of each fiscal quarter, a ratio of total debt to total capitalization of no greater than 70 percent.
Ratio of Total Debt to Total Capital	52.00%
Debt Instrument Covenant Description	In addition to these financial covenants, our credit facilities and public indentures contain usual and customary covenants for our business, including covenants substantially limiting liens, substantial asset sales and mergers. Additionally, our public debt indentures relating to our senior notes and debentures, as well as our revolving credit agreements, each contain a default provision that is triggered if outstanding indebtedness arising out of any other credit agreements in amounts ranging from in excess of $15 million to in excess of $100 million becomes due by acceleration or is not paid at maturity.
Debt Instrument Covenant Compliance	We were in compliance with all of our debt covenants as of June 30, 2013.
First Facility Regulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	950,000,000	750,000,000
Line of Credit Facility, Maximum Borrowing Capacity with Accordion Feature	1,200,000,000
Second Facility Regulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	25,000,000
Third Facility Regulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	14,000,000
Line of Credit Facility, Current Available Borrowing Capacity	8,200,000
Intercompany Facility Regulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	500,000,000
Line Of Credit Facility Description	which bears interest at the lower of (i) the Eurodollar rate under the five-year revolving credit facility or (ii) the lowest rate outstanding under the commercial paper program
Committed Facility Nonregulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	25,000,000	200,000,000
Line of Credit Facility, Maximum Borrowing Capacity with Accordion Feature		500,000,000
Uncommitted Facility Nonregulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	25,000,000
Intercompany Facility Nonregulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	500,000,000
Line Of Credit Facility Description	This facility bears interest at a rate equal to the greater of (i) the one-month LIBOR rate plus 3.00 percent or (ii) the rate for AEM’s borrowings under its committed credit facility plus 0.75 percent.
Nonregulated Facilities [Member]
Line Of Credit Facility [Line Items]
Line of Credit Facility, Current Available Borrowing Capacity	38,600,000
Regulated Facilities [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	$ 989,000,000