Retirement and Post-Retirement Employee Benefit Plans Defined Contribution Plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Schedule of Defined Contribution Plans Disclosures [Line Items]
Defined Contribution, Employer Contribution Service Period Until Eligibility	1 year
Percent of Atmos Energy common stock in Retirement Savings Plan	4.50%	4.90%
Retirement Savings Plan [Member]
Schedule of Defined Contribution Plans Disclosures [Line Items]
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent	65.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	4.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	100.00%
Union 401K Plan [Member]
Schedule of Defined Contribution Plans Disclosures [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	6.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	50.00%
Retirement Savings Plan and Union 401K Plan [Member]
Schedule of Defined Contribution Plans Disclosures [Line Items]
Defined Contribution Plan, Cost Recognized	$ 10.9	$ 10.4	$ 10.5
AEH 401K Profit-Sharing Plan [Member]
Schedule of Defined Contribution Plans Disclosures [Line Items]
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent	75.00%
Defined Contribution Plan, Employer Discretionary Contribution Amount	$ 1.4	$ 1.1	$ 1.2