Debt (Details) (USD $)	3 Months Ended	9 Months Ended
	Jun. 30, 2011	Jun. 30, 2011	Sep. 30, 2010	Jun. 30, 2010
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	$ 2,212,630,000	$ 2,212,630,000	$ 2,172,696,000
Debt Instrument Unamortized Discount	(4,090,000)	(4,090,000)	(3,014,000)
Current maturities of long-term debt	2,434,000	2,434,000	360,131,000
Total long-term debt	2,206,106,000	2,206,106,000	1,809,551,000
Long Term Debt Other Disclosures [Abstract]
Authorized Commercial Paper	750,000,000	750,000,000
Commercial Paper	248,000,000	248,000,000		126,100,000
Ratio of Total Debt to Total Capital	51.00%	51.00%
Letters of Credit Outstanding, Amount	24,800,000	24,800,000
Nonregulated Operations Line of Credit Facilities, Covenant Terms	Additionally, AEM must maintain minimum levels of net working capital and net worth ranging from $20 million to $40 million. 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.
Ratio of Liabilities to Tangible Net Worth	1.34 to 1	1.34 to 1
AEM Net Working Capital	139,500,000	139,500,000
AEM Tangible Net Worth	150,900,000	150,900,000
Debt and Equity Securities Authorized for Issuance	1,300,000,000	1,300,000,000
Debt Securities Authorized for Issuance	950,000,000	950,000,000
Equity Securities Authorized for Issuance	350,000,000	350,000,000
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 Baa1. 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	900,000,000
Debt Securities Available for Issuance	550,000,000	550,000,000
Equity Securities Available for Issuance	350,000,000	350,000,000
Unsecured Senior Notes Due May 2011 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	0	0	350,000,000
Debt Instrument Interest Rate Stated Percentage	7.375%	7.375%
Debt Instrument Maturity Date	May 15, 2011
Unsecured Notes Due December 2011 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	2,303,000	2,303,000	2,303,000
Debt Instrument Interest Rate Stated Percentage	10.00%	10.00%
Debt Instrument Maturity Date	Dec. 31, 2011
Unsecured Senior Notes Due 2013 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	250,000,000	250,000,000	250,000,000
Debt Instrument Interest Rate Stated Percentage	5.125%	5.125%
Debt Instrument Maturity Date	Jan. 15, 2013
Unsecured Senior Notes Due 2014 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	500,000,000	500,000,000	500,000,000
Debt Instrument Interest Rate Stated Percentage	4.95%	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	250,000,000
Debt Instrument Interest Rate Stated Percentage	6.35%	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	450,000,000
Debt Instrument Interest Rate Stated Percentage	8.50%	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	200,000,000
Debt Instrument Interest Rate Stated Percentage	5.95%	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	0
Debt Instrument Interest Rate Stated Percentage	5.50%	5.50%
Debt Instrument Maturity Date	Jun. 15, 2041
Debt Instrument Interest Rate Effective Percentage	5.381%	5.381%
Net Proceeds From Issuance of Debt	394,000,000
Medium Term Notes Due December 2010 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	0	0	10,000,000
Debt Instrument Interest Rate Stated Percentage	6.27%	6.27%
Debt Instrument Maturity Date	Dec. 19, 2010
Medium Term Notes Due 2025 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	10,000,000	10,000,000	10,000,000
Debt Instrument Interest Rate Stated Percentage	6.67%	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	150,000,000
Debt Instrument Interest Rate Stated Percentage	6.75%	6.75%
Debt Instrument Maturity Date	Jul. 15, 2028
Rental Property Term Note [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	327,000	327,000	393,000
Debt Instrument Maturity Date	Jul. 01, 2013
First Facility Regulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	750,000,000	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 2 percent, based on the Company’s credit ratings.
Line Of Credit Facility Amount Outstanding	0	0
Line of Credit Facility, Maximum Borrowing Capacity with Accordion Feature	1,000,000,000	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	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	0
Intercompany Facility Regulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	350,000,000	350,000,000
Line Of Credit Facility Expiration Date	December 31, 2011
Line Of Credit Facility Interest Rate Description	the lower of (i) the one-month LIBOR rate plus 0.45 percent or (ii) the marginal borrowing rate available to the Company on the date of borrowing. The marginal borrowing rate is defined as the lower of (i) a rate based upon the lower of the Prime Rate or the Eurodollar rate under the five year revolving credit facility or (ii) the lowest rate outstanding under the commercial paper program.
Line Of Credit Facility Amount Outstanding	173,800,000	173,800,000
Committed Facility Nonregulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	200,000,000	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	0
Line of Credit Facility, Current Available Borrowing Capacity	125,200,000	125,200,000
Line of Credit Facility, Maximum Borrowing Capacity with Accordion Feature	500,000,000	500,000,000
Line of Credit Facility, Swing Line Dollar Advance Cap, Higher Available Amount Range	30,000,000	30,000,000
Line of Credit Facility, Swing Line Dollar Advance Cap, Lower Available Amount Range	6,000,000	6,000,000
Intercompany Facility Nonregulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity	350,000,000	350,000,000
Line Of Credit Facility Expiration Date	December 31, 2011
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	$ 0