Retirement and Post-Retirement Employee Benefit Plans Defined Contribution Plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2009
Defined Contribution Pension And Other Postretirement Plans Disclosure Abstract
Retirement Savings Plan and Union 401K matching contributions expense	$ 10.2	$ 9.8	$ 9.3
Percent of Atmos Energy common stock in Retirement Savings Plan	4.50%	4.30%
AEM 401K Profit-Sharing Plan Discretionary Contributions	$ 1.3	$ 1.3	$ 1.2

The amount of the cost recognized during the period for defined contribution plans.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 11

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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration