Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015 segment	Mar. 31, 2014	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]
Number of operating segments			3us-gaap_NumberOfOperatingSegments
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	$ 1,540,068	$ 1,964,854	$ 2,798,833	$ 3,208,712
Purchased gas cost	1,019,330us-gaap_CostOfNaturalGasPurchases	1,468,577us-gaap_CostOfNaturalGasPurchases	1,854,810us-gaap_CostOfNaturalGasPurchases	2,323,478us-gaap_CostOfNaturalGasPurchases
Gross profit	520,738us-gaap_GrossProfit	496,277us-gaap_GrossProfit	944,023us-gaap_GrossProfit	885,234us-gaap_GrossProfit
Operating expenses
Operation and maintenance	133,460	124,675	252,042	240,432
Depreciation and amortization	68,022us-gaap_DepreciationAndAmortization	61,307us-gaap_DepreciationAndAmortization	135,615us-gaap_DepreciationAndAmortization	121,776us-gaap_DepreciationAndAmortization
Taxes, other than income	69,046us-gaap_TaxesOther	60,215us-gaap_TaxesOther	118,431us-gaap_TaxesOther	102,226us-gaap_TaxesOther
Total operating expenses	270,528us-gaap_OperatingExpenses	246,197us-gaap_OperatingExpenses	506,088us-gaap_OperatingExpenses	464,434us-gaap_OperatingExpenses
Operating income	250,210us-gaap_OperatingIncomeLoss	250,080us-gaap_OperatingIncomeLoss	437,935us-gaap_OperatingIncomeLoss	420,800us-gaap_OperatingIncomeLoss
Miscellaneous income (expense)	(1,561)us-gaap_OtherNonoperatingIncomeExpense	(1,516)us-gaap_OtherNonoperatingIncomeExpense	(3,268)us-gaap_OtherNonoperatingIncomeExpense	(3,648)us-gaap_OtherNonoperatingIncomeExpense
Interest charges	27,447us-gaap_InterestExpense	31,601us-gaap_InterestExpense	57,211us-gaap_InterestExpense	63,716us-gaap_InterestExpense
Income (loss) before income taxes	221,202	216,963	377,456	353,436
Income tax expense (benefit)	83,518us-gaap_IncomeTaxExpenseBenefit	83,596us-gaap_IncomeTaxExpenseBenefit	142,177us-gaap_IncomeTaxExpenseBenefit	133,053us-gaap_IncomeTaxExpenseBenefit
Net income (loss)	137,684us-gaap_NetIncomeLoss	133,367us-gaap_NetIncomeLoss	235,279us-gaap_NetIncomeLoss	220,383us-gaap_NetIncomeLoss
Capital expenditures	180,331	178,442	441,644	359,009
ASSETS
Net property, plant and equipment	7,026,078us-gaap_PropertyPlantAndEquipmentNet		7,026,078us-gaap_PropertyPlantAndEquipmentNet		6,725,906us-gaap_PropertyPlantAndEquipmentNet
Investment in subsidiaries	0ato_InvestmentinSubsidiaries		0ato_InvestmentinSubsidiaries		0ato_InvestmentinSubsidiaries
Current assets
Cash and cash equivalents	95,525	136,740	95,525	136,740	42,258	66,199
Assets from risk management activities	16,947us-gaap_DerivativeAssetsCurrent		16,947us-gaap_DerivativeAssetsCurrent		45,827us-gaap_DerivativeAssetsCurrent
Other current assets	705,165		705,165		687,755
Intercompany receivables	0ato_IntercompanyReceivables		0ato_IntercompanyReceivables		0ato_IntercompanyReceivables
Total current assets	817,637us-gaap_AssetsCurrent		817,637us-gaap_AssetsCurrent		775,840us-gaap_AssetsCurrent
Goodwill	742,029us-gaap_Goodwill		742,029us-gaap_Goodwill		742,029us-gaap_Goodwill
Noncurrent assets from risk management activities					13,038us-gaap_DerivativeAssetsNoncurrent
Deferred charges and other assets	340,900		340,900		337,891
Total Assets	8,926,644us-gaap_Assets		8,926,644us-gaap_Assets		8,594,704us-gaap_Assets
CAPITALIZATION AND LIABILITIES
Shareholders' equity	3,139,694us-gaap_StockholdersEquity		3,139,694us-gaap_StockholdersEquity		3,086,232us-gaap_StockholdersEquity
Long-term debt	2,455,217us-gaap_LongTermDebtNoncurrent		2,455,217us-gaap_LongTermDebtNoncurrent		2,455,986us-gaap_LongTermDebtNoncurrent
Total capitalization	5,594,911		5,594,911		5,542,218
Current liabilities
Short-term debt	224,986us-gaap_ShortTermBorrowings		224,986us-gaap_ShortTermBorrowings		196,695us-gaap_ShortTermBorrowings
Liabilities from risk management activities	5,769us-gaap_DerivativeLiabilitiesCurrent		5,769us-gaap_DerivativeLiabilitiesCurrent		1,730us-gaap_DerivativeLiabilitiesCurrent
Other current liabilities	787,747		787,747		712,225
Intercompany payables	0ato_IntercompanyPayables		0ato_IntercompanyPayables		0ato_IntercompanyPayables
Total current liabilities	1,018,502us-gaap_LiabilitiesCurrent		1,018,502us-gaap_LiabilitiesCurrent		910,650us-gaap_LiabilitiesCurrent
Deferred income taxes	1,338,755us-gaap_DeferredIncomeTaxLiabilities		1,338,755us-gaap_DeferredIncomeTaxLiabilities		1,286,616us-gaap_DeferredIncomeTaxLiabilities
Derivative Liability, Noncurrent	132,305us-gaap_DerivativeLiabilitiesNoncurrent		132,305us-gaap_DerivativeLiabilitiesNoncurrent		20,126us-gaap_DerivativeLiabilitiesNoncurrent
Regulatory cost of removal obligation	441,655ato_RegulatoryCostOfRemovalObligation		441,655ato_RegulatoryCostOfRemovalObligation		445,387ato_RegulatoryCostOfRemovalObligation
Pension and postretirement liabilities	350,889		350,889		340,963
Deferred credits and other liabilities	49,627		49,627		48,744
Total Equity and Liabilities	8,926,644		8,926,644		8,594,704
Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	1,540,068	1,964,854	2,798,833	3,208,712
Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	0	0	0	0
Regulated Distribution Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	1,130,613	1,290,960	1,977,385	2,134,825
Purchased gas cost	724,378	905,772	1,247,338	1,450,466
Gross profit	406,235	385,188	730,047	684,359
Operating expenses
Operation and maintenance	103,425	106,776	190,410	196,439
Depreciation and amortization	55,153	50,020	110,239	99,571
Taxes, other than income	62,939	60,606	106,583	97,690
Total operating expenses	221,517	217,402	407,232	393,700
Operating income	184,718	167,786	322,815	290,659
Miscellaneous income (expense)	(937)	97	(2,266)	(374)
Interest charges	19,313	22,828	40,953	46,153
Income (loss) before income taxes	164,468	145,055	279,596	244,132
Income tax expense (benefit)	62,615	56,312	106,356	92,632
Net income (loss)	101,853	88,743	173,240	151,500
Capital expenditures	145,990	139,555	312,237	267,061
ASSETS
Net property, plant and equipment	5,401,722		5,401,722		5,202,761
Investment in subsidiaries	983,075		983,075		952,171
Current assets
Cash and cash equivalents	82,732		82,732		33,303
Assets from risk management activities	364		364		23,102
Other current assets	514,627		514,627		490,408
Intercompany receivables	814,495		814,495		790,442
Total current assets	1,412,218		1,412,218		1,337,255
Goodwill	574,816		574,816		574,816
Noncurrent assets from risk management activities					13,038
Deferred charges and other assets	320,918		320,918		309,965
Total Assets	8,692,749		8,692,749		8,390,006
CAPITALIZATION AND LIABILITIES
Shareholders' equity	3,139,694		3,139,694		3,086,232
Long-term debt	2,455,217		2,455,217		2,455,986
Total capitalization	5,594,911		5,594,911		5,542,218
Current liabilities
Short-term debt	529,586		529,586		522,695
Liabilities from risk management activities	5,769		5,769		1,730
Other current liabilities	649,355		649,355		559,765
Intercompany payables	0		0		0
Total current liabilities	1,184,710		1,184,710		1,084,190
Deferred income taxes	948,589		948,589		913,260
Derivative Liability, Noncurrent	132,305		132,305		20,126
Regulatory cost of removal obligation	441,655		441,655		445,387
Pension and postretirement liabilities	350,889		350,889		340,963
Deferred credits and other liabilities	39,690		39,690		43,862
Total Equity and Liabilities	8,692,749		8,692,749		8,390,006
Regulated Distribution Segment [Member] \| Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	1,128,473	1,289,429	1,973,877	2,131,861
Regulated Distribution Segment [Member] \| Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	2,140	1,531	3,508	2,964
Regulated Pipeline Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	91,730	73,615	175,297	144,956
Purchased gas cost	0	0	0	0
Gross profit	91,730	73,615	175,297	144,956
Operating expenses
Operation and maintenance	22,842	16,595	47,457	33,895
Depreciation and amortization	11,747	10,156	23,129	19,942
Taxes, other than income	5,238	(1,232)	10,103	3,431
Total operating expenses	39,827	25,519	80,689	57,268
Operating income	51,903	48,096	94,608	87,688
Miscellaneous income (expense)	(379)	(1,081)	(631)	(2,262)
Interest charges	8,391	9,155	16,715	18,112
Income (loss) before income taxes	43,133	37,860	77,262	67,314
Income tax expense (benefit)	15,451	13,751	27,545	23,759
Net income (loss)	27,682	24,109	49,717	43,555
Capital expenditures	34,360	39,000	129,114	91,921
ASSETS
Net property, plant and equipment	1,567,618		1,567,618		1,464,572
Investment in subsidiaries	0		0		0
Current assets
Cash and cash equivalents	0		0		0
Assets from risk management activities	0		0		0
Other current assets	13,710		13,710		14,009
Intercompany receivables	0		0		0
Total current assets	13,710		13,710		14,009
Goodwill	132,502		132,502		132,502
Noncurrent assets from risk management activities					0
Deferred charges and other assets	14,592		14,592		21,826
Total Assets	1,728,422		1,728,422		1,632,909
CAPITALIZATION AND LIABILITIES
Shareholders' equity	532,329		532,329		482,612
Long-term debt	0		0		0
Total capitalization	532,329		532,329		482,612
Current liabilities
Short-term debt	0		0		0
Liabilities from risk management activities	0		0		0
Other current liabilities	13,129		13,129		24,790
Intercompany payables	783,147		783,147		763,635
Total current liabilities	796,276		796,276		788,425
Deferred income taxes	398,589		398,589		361,688
Derivative Liability, Noncurrent	0		0		0
Regulatory cost of removal obligation	0		0		0
Pension and postretirement liabilities	0		0		0
Deferred credits and other liabilities	1,228		1,228		184
Total Equity and Liabilities	1,728,422		1,728,422		1,632,909
Regulated Pipeline Segment [Member] \| Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	24,477	21,002	45,028	42,172
Regulated Pipeline Segment [Member] \| Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	67,253	52,613	130,269	102,784
Nonregulated Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	438,322	758,215	900,610	1,194,646
Purchased gas cost	415,416	720,626	861,665	1,138,491
Gross profit	22,906	37,589	38,945	56,155
Operating expenses
Operation and maintenance	7,326	1,419	14,441	10,334
Depreciation and amortization	1,122	1,131	2,247	2,263
Taxes, other than income	869	841	1,745	1,105
Total operating expenses	9,317	3,391	18,433	13,702
Operating income	13,589	34,198	20,512	42,453
Miscellaneous income (expense)	252	443	552	767
Interest charges	240	593	466	1,230
Income (loss) before income taxes	13,601	34,048	20,598	41,990
Income tax expense (benefit)	5,452	13,533	8,276	16,662
Net income (loss)	8,149	20,515	12,322	25,328
Capital expenditures	(19)	(113)	293	27
ASSETS
Net property, plant and equipment	56,738		56,738		58,573
Investment in subsidiaries	(2,096)		(2,096)		(2,096)
Current assets
Cash and cash equivalents	12,793		12,793		8,955
Assets from risk management activities	16,583		16,583		22,725
Other current assets	497,411		497,411		526,161
Intercompany receivables	0		0		0
Total current assets	526,787		526,787		557,841
Goodwill	34,711		34,711		34,711
Noncurrent assets from risk management activities					0
Deferred charges and other assets	5,390		5,390		6,100
Total Assets	621,530		621,530		655,129
CAPITALIZATION AND LIABILITIES
Shareholders' equity	450,746		450,746		469,559
Long-term debt	0		0		0
Total capitalization	450,746		450,746		469,559
Current liabilities
Short-term debt	0		0		0
Liabilities from risk management activities	0		0		0
Other current liabilities	139,150		139,150		142,397
Intercompany payables	31,348		31,348		26,807
Total current liabilities	170,498		170,498		169,204
Deferred income taxes	(8,423)		(8,423)		11,668
Derivative Liability, Noncurrent	0		0		0
Regulatory cost of removal obligation	0		0		0
Pension and postretirement liabilities	0		0		0
Deferred credits and other liabilities	8,709		8,709		4,698
Total Equity and Liabilities	621,530		621,530		655,129
Nonregulated Segment [Member] \| Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	387,118	654,423	779,928	1,034,679
Nonregulated Segment [Member] \| Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	51,204	103,792	120,682	159,967
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	(120,597)	(157,936)	(254,459)	(265,715)
Purchased gas cost	(120,464)	(157,821)	(254,193)	(265,479)
Gross profit	(133)	(115)	(266)	(236)
Operating expenses
Operation and maintenance	(133)	(115)	(266)	(236)
Depreciation and amortization	0	0	0	0
Taxes, other than income	0	0	0	0
Total operating expenses	(133)	(115)	(266)	(236)
Operating income	0	0	0	0
Miscellaneous income (expense)	(497)	(975)	(923)	(1,779)
Interest charges	(497)	(975)	(923)	(1,779)
Income (loss) before income taxes	0	0	0	0
Income tax expense (benefit)	0	0	0	0
Net income (loss)	0	0	0	0
Capital expenditures	0	0	0	0
ASSETS
Net property, plant and equipment	0		0		0
Investment in subsidiaries	(980,979)		(980,979)		(950,075)
Current assets
Cash and cash equivalents	0		0		0
Assets from risk management activities	0		0		0
Other current assets	(320,583)		(320,583)		(342,823)
Intercompany receivables	(814,495)		(814,495)		(790,442)
Total current assets	(1,135,078)		(1,135,078)		(1,133,265)
Goodwill	0		0		0
Noncurrent assets from risk management activities					0
Deferred charges and other assets	0		0		0
Total Assets	(2,116,057)		(2,116,057)		(2,083,340)
CAPITALIZATION AND LIABILITIES
Shareholders' equity	(983,075)		(983,075)		(952,171)
Long-term debt	0		0		0
Total capitalization	(983,075)		(983,075)		(952,171)
Current liabilities
Short-term debt	(304,600)		(304,600)		(326,000)
Liabilities from risk management activities	0		0		0
Other current liabilities	(13,887)		(13,887)		(14,727)
Intercompany payables	(814,495)		(814,495)		(790,442)
Total current liabilities	(1,132,982)		(1,132,982)		(1,131,169)
Deferred income taxes	0		0		0
Derivative Liability, Noncurrent	0		0		0
Regulatory cost of removal obligation	0		0		0
Pension and postretirement liabilities	0		0		0
Deferred credits and other liabilities	0		0		0
Total Equity and Liabilities	(2,116,057)		(2,116,057)		(2,083,340)
Intersegment Elimination [Member] \| Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	0	0	0	0
Intersegment Elimination [Member] \| Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	$ (120,597)	$ (157,936)	$ (254,459)	$ (265,715)

Amounts due to other operating segments of the same entity attributed to the reportable segment.

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Amounts due from other operating segments of the same entity attributed to the reportable segment.

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Aggregate carrying amount, as of the balance sheet date, of obligations incurred and payable, pertaining to costs that are statutory in nature, are incurred on contractual obligations, or accumulate over time and for which invoices have not yet been received or will not be rendered and of liabilities not separately disclosed in the segment balance sheet. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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Sum of the carrying amounts as of the balance sheet date of all assets that are expected to be realized in cash, sold, or consumed within one year (or the normal operating cycle, if longer). Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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This element represents the total consolidated (as applicable) capitalization of the entity which is comprised of its long-term debt and equity instruments. The table may be detailed by subsidiary (legal entity) and include information by type of debt or equity detailed by instrument.

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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