Debt (Details) (USD $)	12 Months Ended					12 Months Ended		12 Months Ended					0 Months Ended				12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		3 Months Ended	12 Months Ended		12 Months Ended			12 Months Ended		12 Months Ended		12 Months Ended
	Sep. 30, 2013 credit_facility	Sep. 30, 2012	Sep. 30, 2013 Minimum [Member]	Sep. 30, 2013 Maximum [Member]	Sep. 30, 2013 Regulated Facilities [Member]	Sep. 30, 2013 First Facility Regulated Operations [Member]	Sep. 30, 2012 First Facility Regulated Operations [Member]	Sep. 30, 2013 Second Facility Regulated Operations [Member]	Sep. 30, 2013 Third Facility Regulated Operations [Member]	Sep. 30, 2013 Intercompany Facility Regulated Operations [Member]	Sep. 30, 2013 Nonregulated Facilities [Member]	Dec. 05, 2012 Nonregulated Facilities [Member] credit_facility	Dec. 05, 2012 Committed Facility Nonregulated Operations [Member]	Sep. 30, 2013 Committed Facility Nonregulated Operations [Member]	Sep. 30, 2012 Committed Facility Nonregulated Operations [Member]	Sep. 30, 2013 Uncommitted Facility Nonregulated Operations [Member] credit_facility	Sep. 30, 2013 Intercompany Facility Nonregulated Operations [Member]	Sep. 30, 2012 Intercompany Facility Nonregulated Operations [Member]	Sep. 30, 2013 Unsecured Senior Notes Due 2014 [Member]	Sep. 30, 2012 Unsecured Senior Notes Due 2014 [Member]	Sep. 30, 2013 Unsecured Senior Notes Due 2017 [Member]	Sep. 30, 2012 Unsecured Senior Notes Due 2017 [Member]	Sep. 30, 2013 Unsecured Senior Notes Due 2019 [Member]	Sep. 30, 2012 Unsecured Senior Notes Due 2019 [Member]	Sep. 30, 2013 Unsecured Senior Notes Due 2034 [Member]	Sep. 30, 2012 Unsecured Senior Notes Due 2034 [Member]	Sep. 30, 2013 Unsecured Senior Notes Due 2041 [Member]	Sep. 30, 2012 Unsecured Senior Notes Due 2041 [Member]	Sep. 30, 2013 Unsecured Senior Notes Due 2043 [Member]	Sep. 30, 2013 Unsecured Senior Notes Due 2043 [Member]	Sep. 30, 2012 Unsecured Senior Notes Due 2043 [Member]	Sep. 30, 2013 Bridge Loan [Member]	Sep. 30, 2013 Medium Term Notes Due 2025 [Member]	Sep. 30, 2012 Medium Term Notes Due 2025 [Member]	Sep. 30, 2013 Unsecured Debentures Due 2028 [Member]	Sep. 30, 2012 Unsecured Debentures Due 2028 [Member]	Sep. 30, 2013 Rental Property Term Note [Member]	Sep. 30, 2012 Rental Property Term Note [Member]	Sep. 30, 2012 Unsecured Senior Notes Due 2013 [Member]
Debt Instrument [Line Items]
Debt Instrument Interest Rate Stated Percentage																			4.95%		6.35%		8.50%		5.95%		5.50%		4.15%	4.15%			6.67%		6.75%
Debt Instrument Maturity Date																			Oct. 15, 2014		Jun. 15, 2017		Mar. 15, 2019		Oct. 15, 2034		Jun. 15, 2041			Jan. 15, 2043			Dec. 15, 2025		Jul. 15, 2028		Jul. 01, 2013
Debt Instrument Carrying Amount	$ 2,460,000,000	$ 1,960,131,000																	$ 500,000,000	$ 500,000,000	$ 250,000,000	$ 250,000,000	$ 450,000,000	$ 450,000,000	$ 200,000,000	$ 200,000,000	$ 400,000,000	$ 400,000,000	$ 500,000,000	$ 500,000,000	$ 0		$ 10,000,000	$ 10,000,000	$ 150,000,000	$ 150,000,000	$ 0	$ 131,000
Debt Instrument Unamortized Discount	4,329,000	3,695,000
Current maturities of long-term debt	0	131,000
Long-term debt	2,455,671,000	1,956,305,000
Debt Instrument Interest Rate Effective Percentage																													4.67%	4.67%
Net Proceeds From Issuance of Debt																													494,000,000
Remaining Proceeds From Issuance Of Debt																														234,000,000
Extinguishment of Debt, Amount																																260,000,000							250,000,000
Extinguishment of Debt, Interest Terms	LIBOR based rate plus a company specific spread,																																						0.05125
Line Of Credit Facility [Line Items]
Number of Credit Facilities	4											2				1
Line Of Credit Facility Maximum Borrowing Capacity	1,000,000,000				985,000,000	950,000,000	750,000,000	25,000,000	10,000,000	500,000,000				25,000,000	200,000,000	25,000,000	500,000,000
Line of Credit Facility, Term						5 years					364 days		3 years
Line Of Credit Facility Interest Rate Description						a base rate or at a LIBOR-based rate for the applicable interest period, plus a spread ranging from zero percent to two percent, based on the Company’s credit ratings		a daily negotiated rate, generally based on the Federal Funds rate plus a variable margin	LIBOR-based rate plus 1.5 percent.	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							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 line of credit facility plus 0.75 percent
Line Of Credit Facility Amount Outstanding						0		0	0	278,000,000							0
Line of Credit Facility, Current Available Borrowing Capacity						582,000,000					37,400,000
Line of Credit Facility, Maximum Borrowing Capacity with Accordion Feature						1,200,000,000												500,000,000
Debt Instrument, Covenant, Excess Debt Trigger			15,000,000	100,000,000
Long-term Debt, Other Disclosures [Abstract]
Authorized Commercial Paper	950,000,000
Commercial Paper	368,000,000	310,900,000
Commercial Paper Weighted Average Interest Rate	0.25%	0.48%
Debt Covenant, Total Debt To Total Capital Ratio	70.00%
Ratio of Total Debt to Total Capital	54.00%
Regulated Operations Letters of Credit Outstanding, Amount	5,900,000
Debt and Equity Securities Authorized for Issuance	1,750,000,000
Debt Instrument Covenant Compliance	We were in compliance with all of our debt covenants as of September 30, 2013
Debt And Equity Securities Available For Issuance	1,750,000,000
Long-term Debt, Fiscal Year Maturity [Abstract]
2014	0
2015	500,000,000
2016	0
2017	250,000,000
2018	0
Thereafter	1,710,000,000
Debt Instrument Carrying Amount	$ 2,460,000,000	$ 1,960,131,000																	$ 500,000,000	$ 500,000,000	$ 250,000,000	$ 250,000,000	$ 450,000,000	$ 450,000,000	$ 200,000,000	$ 200,000,000	$ 400,000,000	$ 400,000,000	$ 500,000,000	$ 500,000,000	$ 0		$ 10,000,000	$ 10,000,000	$ 150,000,000	$ 150,000,000	$ 0	$ 131,000