Debt (Details) (USD $)	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	$ 1,960,131,000	$ 2,212,565,000
Debt Instrument Unamortized Discount	(3,695,000)	(4,014,000)
Current maturities of long-term debt	131,000	2,434,000
Total long-term debt	1,956,305,000	2,206,117,000
Long Term Debt By Maturity Abstract
Long Term Debt Maturities Repayments Of Principal In Next Twelve Months	131,000
Long Term Debt Maturities Repayments Of Principal In Year Two	0
Long Term Debt Maturities Repayments Of Principal In Year Three	500,000,000
Long Term Debt Maturities Repayments Of Principal In Year Four	0
Long Term Debt Maturities Repayments Of Principal In Year Five	250,000,000
Long Term Debt Maturities Repayments Of Principal After Year Five	1,210,000,000
Total Debt Instrument Carrying Amount	1,960,131,000	2,212,565,000
Long Term Debt Other Disclosures [Abstract]
Authorized Commercial Paper	750,000,000
Commercial Paper	310,900,000	206,400,000
Commercial Paper Weighted Average Interest Rate	0.43%
Regulated Operations Line of Credit Facilities, Covenant Terms	The availability of funds under these credit facilities is subject to conditions specified in the respective credit agreements, all of which we currently satisfy. These conditions include our compliance with financial covenants and the continued accuracy of representations and warranties contained in these agreements. 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	54.00%
Regulated Operations Letters of Credit Outstanding, Amount	11,500,000
Nonregulated Operations Letters of Credit Outstanding, Amount	11,500,000
Nonregulated Operations Line of Credit Facilities, Covenant Terms	AEM is required by the financial covenants in this facility to maintain a ratio of total liabilities to tangible net worth that does not exceed a maximum of 5 to 1. Additionally, AEM must maintain minimum levels of net working capital and net worth ranging from $20 million to $40 million.
Ratio of Liabilities to Tangible Net Worth	0.74 to 1
AEM Net Working Capital	136,200,000
AEM Tangible Net Worth	150,800,000
Debt and Equity Securities Authorized for Issuance	1,300,000,000
Debt Instrument Covenant Description	In addition to the financial covenants described above, 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. Further, AEM’s credit agreement contains a cross-default provision whereby AEM would be in default if it defaults on other indebtedness, as defined, by at least $250 thousand in the aggregate. Finally, AEM’s credit agreement contains a provision that would limit the amount of credit available if Atmos Energy were downgraded below an S&P rating of BBB and a Moody’s rating of Baa2. We have no other triggering events in our debt instruments that are tied to changes in specified credit ratings or stock price, nor have we entered into any transactions that would require us to issue equity, based on our credit rating or other triggering events.
Debt Instrument Covenant Compliance	We were in compliance with all of our debt covenants
Debt And Equity Securities Available For Issuance	900,000,000
First Facility Regulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	750,000,000
Line Of Credit Facility Expiration Date	May 2, 2016
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
Line Of Credit Facility Amount Outstanding	0
Line of Credit Facility, Current Available Borrowing Capacity	439,100,000
Line of Credit Facility, Maximum Borrowing Capacity with Accordion Feature	1,000,000,000
Second Facility Regulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	25,000,000
Line Of Credit Facility Interest Rate Description	a daily negotiated rate, generally based on the Federal Funds rate plus a variable margin
Line Of Credit Facility Amount Outstanding	0
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 Expiration Date	September 30, 2013
Line Of Credit Facility Amount Outstanding	0
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 Expiration Date	December 31, 2012
Line Of Credit Facility Amount Outstanding	211,500,000
Committed Facility Nonregulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	200,000,000
Line Of Credit Facility Interest Rate Description	a base rate or an offshore rate, in each case plus an applicable margin. The base rate is a floating rate equal to the higher of: (a) 0.50 percent per annum above the latest Federal Funds rate; (b) the per annum rate of interest established by BNP Paribas from time to time as its “prime rate” or “base rate” for U.S. dollar loans; (c) an offshore rate (based on LIBOR with a three-month interest period) as in effect from time to time; or (d) the “cost of funds” rate which is the cost of funds as reasonably determined by the administrative agent. The offshore rate is a floating rate equal to the higher of (a) an offshore rate based upon LIBOR for the applicable interest period; or (b) a “cost of funds” rate referred to above. In the case of both base rate and offshore rate loans, the applicable margin ranges from 1.875 percent to 2.25 percent per annum, depending on the excess tangible net worth of AEM, as defined in the credit facility
Line Of Credit Facility Amount Outstanding	0
Line of Credit Facility, Current Available Borrowing Capacity	138,500,000
Line of Credit Facility, Maximum Borrowing Capacity with Accordion Feature	500,000,000
Line of Credit Facility, Swing Line Dollar Advance Cap, Higher Available Amount Range	30,000,000
Line of Credit Facility, Swing Line Dollar Advance Cap, Lower Available Amount Range	6,000,000
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 Expiration Date	December 31, 2012
Line Of Credit Facility Interest Rate Description	a rate equal to the greater of (i) the one-month LIBOR rate plus 3.00 percent or (ii) the rate for AEM’s offshore borrowings under its committed credit facility plus 0.75 percent
Line Of Credit Facility Amount Outstanding	0
Short-term Financing Facility [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	260,000,000
Line Of Credit Facility Expiration Date	February 1, 2013
Line Of Credit Facility Amount Outstanding	260,000,000
Unsecured Notes Due December 2011 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	0	2,303,000
Debt Instrument Interest Rate Stated Percentage	10.00%
Debt Instrument Maturity Date	Dec. 31, 2011
Long Term Debt By Maturity Abstract
Total Debt Instrument Carrying Amount	0	2,303,000
Unsecured Senior Notes Due 2013 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	0	250,000,000
Debt Instrument Interest Rate Stated Percentage	5.125%
Debt Instrument Maturity Date	Jan. 15, 2013
Debt Instrument, Early Redemption Date	Aug. 28, 2012
Debt Instrument, Redemption Fee Amount	4,600,000
Long Term Debt By Maturity Abstract
Total Debt Instrument Carrying Amount	0	250,000,000
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
Long Term Debt By Maturity Abstract
Total Debt Instrument Carrying Amount	500,000,000	500,000,000
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
Long Term Debt By Maturity Abstract
Total Debt Instrument Carrying Amount	250,000,000	250,000,000
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
Long Term Debt By Maturity Abstract
Total Debt Instrument Carrying Amount	450,000,000	450,000,000
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
Long Term Debt By Maturity Abstract
Total Debt Instrument Carrying Amount	200,000,000	200,000,000
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
Long Term Debt By Maturity Abstract
Total Debt Instrument Carrying Amount	400,000,000	400,000,000
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
Long Term Debt By Maturity Abstract
Total Debt Instrument Carrying Amount	10,000,000	10,000,000
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
Long Term Debt By Maturity Abstract
Total Debt Instrument Carrying Amount	150,000,000	150,000,000
Rental Property Term Note [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	131,000	262,000
Debt Instrument Maturity Date	Jul. 01, 2013
Long Term Debt By Maturity Abstract
Total Debt Instrument Carrying Amount	$ 131,000	$ 262,000