Debt (Details) (USD $)	3 Months Ended
	Dec. 31, 2012	Sep. 30, 2012
Long Term Debt Other Disclosures [Abstract]
Authorized Commercial Paper	$ 750,000,000
Commercial Paper	570,900,000	310,900,000
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	55.00%
Debt and Equity Securities Authorized for Issuance	1,300,000,000
Debt Instrument Covenant Description	In addition to these financial covenants, our credit facilities and public debt 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 December 31, 2012.
Debt And Equity Securities Available For Issuance	900,000,000
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	1,960,131,000	1,960,131,000
Debt Instrument Unamortized Discount	3,624,000	3,695,000
Current maturities of long-term debt	131,000	131,000
Total long-term debt	1,956,376,000	1,956,305,000
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	950,000,000
First Facility Regulated Operations [Member]
Line Of Credit Facility [Line Items]
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	2,500,000
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
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.
Short Term Financing Facility [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Amount Outstanding	260,000,000
Uncommitted Facility Nonregulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	25,000,000
Unsecured Senior Notes Due 2014 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	500,000,000	500,000,000
Debt Instrument Interest Rate Stated Percentage	4.95%
Debt Instrument Maturity Date	Oct. 15, 2014
Unsecured Senior Notes Due 2017 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	250,000,000	250,000,000
Debt Instrument Interest Rate Stated Percentage	6.35%
Debt Instrument Maturity Date	Jun. 15, 2017
Unsecured Senior Notes Due 2019 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	450,000,000	450,000,000
Debt Instrument Interest Rate Stated Percentage	8.50%
Debt Instrument Maturity Date	Mar. 15, 2019
Unsecured Senior Notes Due 2034 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	200,000,000	200,000,000
Debt Instrument Interest Rate Stated Percentage	5.95%
Debt Instrument Maturity Date	Oct. 15, 2034
Unsecured Senior Notes Due 2041 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	400,000,000	400,000,000
Debt Instrument Interest Rate Stated Percentage	5.50%
Debt Instrument Maturity Date	Jun. 15, 2041
Medium Term Notes Due 2025 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	10,000,000	10,000,000
Debt Instrument Interest Rate Stated Percentage	6.67%
Debt Instrument Maturity Date	Dec. 15, 2025
Unsecured Debentures Due 2028 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	150,000,000	150,000,000
Debt Instrument Interest Rate Stated Percentage	6.75%
Debt Instrument Maturity Date	Jul. 15, 2028
Rental Property Term Note [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	$ 131,000	$ 131,000
Debt Instrument Maturity Date	Jul. 01, 2013