Debt (Details) (USD $)	0 Months Ended	12 Months Ended						12 Months Ended		12 Months Ended							12 Months Ended	0 Months Ended	12 Months Ended		0 Months Ended		12 Months Ended								3 Months Ended	12 Months Ended		0 Months Ended		12 Months Ended
	Feb. 18, 2014	Sep. 30, 2014 credit_facility	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2014 Minimum [Member]	Sep. 30, 2014 Maximum [Member]	Sep. 30, 2014 Regulated Facilities [Member]	Sep. 30, 2014 First Facility Regulated Operations [Member]	Sep. 30, 2013 First Facility Regulated Operations [Member]	Sep. 30, 2014 Second Facility Regulated Operations [Member]	Sep. 30, 2014 Third Facility Regulated Operations [Member]	Sep. 30, 2014 Intercompany Facility Regulated Operations [Member]	Sep. 30, 2014 Nonregulated Facilities [Member]	Sep. 30, 2014 Committed Facility Nonregulated Operations [Member]	Sep. 30, 2013 Committed Facility Nonregulated Operations [Member]	Sep. 30, 2014 Uncommitted Facility Nonregulated Operations [Member] credit_facility	Sep. 30, 2014 Intercompany Facility Nonregulated Operations [Member]	Oct. 15, 2014 Subsequent Event [Member]	Sep. 30, 2014 Unsecured Senior Notes Due 2014 [Member]	Sep. 30, 2013 Unsecured Senior Notes Due 2014 [Member]	Oct. 15, 2014 Unsecured Senior Notes Due 2014 [Member] Subsequent Event [Member]	Oct. 15, 2014 Unsecured Senior Notes Due 2014 [Member] Subsequent Event [Member]	Sep. 30, 2014 Unsecured Senior Notes Due 2017 [Member]	Sep. 30, 2013 Unsecured Senior Notes Due 2017 [Member]	Sep. 30, 2014 Unsecured Senior Notes Due 2019 [Member]	Sep. 30, 2013 Unsecured Senior Notes Due 2019 [Member]	Sep. 30, 2014 Unsecured Senior Notes Due 2034 [Member]	Sep. 30, 2013 Unsecured Senior Notes Due 2034 [Member]	Sep. 30, 2014 Unsecured Senior Notes Due 2041 [Member]	Sep. 30, 2013 Unsecured Senior Notes Due 2041 [Member]	Sep. 30, 2013 Unsecured Senior Notes Due 2043 [Member]	Sep. 30, 2014 Unsecured Senior Notes Due 2043 [Member]	Sep. 30, 2013 Unsecured Senior Notes Due 2043 [Member]	Oct. 15, 2014 Unsecured Senior Notes Due 2044 [Member] Subsequent Event [Member]	Oct. 15, 2014 Unsecured Senior Notes Due 2044 [Member] Subsequent Event [Member]	Sep. 30, 2014 Bridge Loan [Member]	Sep. 30, 2014 Medium Term Notes Due 2025 [Member]	Sep. 30, 2013 Medium Term Notes Due 2025 [Member]	Sep. 30, 2014 Unsecured Debentures Due 2028 [Member]	Sep. 30, 2013 Unsecured Debentures Due 2028 [Member]	Sep. 30, 2014 Rental Property Term Note [Member]	Sep. 30, 2013 Rental Property Term Note [Member]	Sep. 30, 2013 Unsecured Senior Notes Due 2013 [Member]
Debt Instrument [Line Items]
Debt Instrument Interest Rate Stated Percentage																			4.95%	4.95%			6.35%	6.35%	8.50%	8.50%	5.95%	5.95%	5.50%	5.50%	4.15%	4.15%	4.15%		4.125%		6.67%	6.67%	6.75%	6.75%
Debt Instrument Maturity Date																			Oct. 15, 2014	Oct. 15, 2014			Jun. 15, 2017	Jun. 15, 2017	Mar. 15, 2019	Mar. 15, 2019	Oct. 15, 2034	Oct. 15, 2034	Jun. 15, 2041	Jun. 15, 2041		Jan. 15, 2043	Jan. 15, 2043				Dec. 15, 2025	Dec. 15, 2025	Jul. 15, 2028	Jul. 15, 2028	Jul. 01, 2013	Jul. 01, 2013
Debt Instrument Carrying Amount		$ 2,460,000,000	$ 2,460,000,000																$ 500,000,000	$ 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	$ 500,000,000		$ 500,000,000		$ 10,000,000	$ 10,000,000	$ 150,000,000	$ 150,000,000
Debt Instrument Unamortized Discount		4,014,000	4,329,000
Current maturities of long-term debt		0	0
Debt Instrument Interest Rate Effective Percentage																																4.67%			4.086%
Net Proceeds From Issuance of Debt																															494,000,000			494,000,000
Remaining Proceeds From Issuance Of Debt																																	234,000,000
Extinguishment of Debt, Amount																					500,000,000															260,000,000							250,000,000
Extinguishment of Debt, Interest Terms																					0.0495																						0.05125
Subsequent Event, Date																		Oct. 15, 2014
Line Of Credit Facility [Line Items]
Number of Credit Facilities		4											2			1
Line Of Credit Facility Maximum Borrowing Capacity		1,300,000,000	25,000,000				1,300,000,000	1,250,000,000	950,000,000	25,000,000	10,000,000	500,000,000		15,000,000	25,000,000	25,000,000	500,000,000
Line of Credit Facility, Term								5 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	326,000,000					0
Line of Credit Facility, Current Available Borrowing Capacity								1,053,300,000					32,200,000
Line of Credit Facility, Maximum Borrowing Capacity with Accordion Feature								1,500,000,000
Debt Instrument, Covenant, Excess Debt Trigger					15,000,000	100,000,000
Regulated Operations Letters of Credit Outstanding, Amount											5,900,000
Long-term Debt, Other Disclosures [Abstract]
Authorized Commercial Paper		1,250,000,000
Commercial Paper		196,700,000	368,000,000
Commercial Paper Weighted Average Interest Rate		0.23%	0.25%
Debt Covenant, Total Debt To Total Capital Ratio		70.00%
Ratio of Total Debt to Total Capital		48.00%
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, 2014
Debt And Equity Securities Available For Issuance		845,000,000
Stock Issued During Period, Shares, New Issues	9,200,000
Net proceeds from equity offering	390,200,000	390,205,000	0	0
Long-term Debt, Fiscal Year Maturity [Abstract]
2015		500,000,000
2016		0
2017		250,000,000
2018		0
2019		450,000,000
Thereafter		1,260,000,000
Debt Instrument Carrying Amount		$ 2,460,000,000	$ 2,460,000,000																$ 500,000,000	$ 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	$ 500,000,000		$ 500,000,000		$ 10,000,000	$ 10,000,000	$ 150,000,000	$ 150,000,000