XML 18 R7.htm IDEA: XBRL DOCUMENT
v3.7.0.1

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) (PARENTHETICALS) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Jun. 30, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 30, 2016
Other Comprehensive Income Loss Tax Portion Attributable To Parent Abstract |  |  |  |
Unrealized holding gains (losses) on available-for-sale securities, tax | $ 490 | $ 110 | $ 893 | $ (837)
Interest Rate Contract [Member] |  |  |  |
Schedule of Other Comprehensive Income (Loss), Tax, Cash Flow Hedges [Line Items] |  |  |  |
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Tax | (10,667) | (22,561) | 44,194 | (50,631)
Commodity Contract [Member] |  |  |  |
Schedule of Other Comprehensive Income (Loss), Tax, Cash Flow Hedges [Line Items] |  |  |  |
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Tax | $ 0 | $ 11,575 | $ 3,183 | $ 13,220

X
- Definition [Line Items] for Schedule of Other Comprehensive Income (Loss), Tax, Cash Flow Hedges [Table] + References No definition available. + Details
Name: | ato_ScheduleofOtherComprehensiveIncomeLossTaxCashFlowHedgesLineItems
Namespace Prefix: | ato_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of tax expense (benefit), after reclassification adjustments, of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=66004124&loc=SL7669646-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=66004124&loc=d3e640-108580 + Details
Name: | us-gaap_OtherComprehensiveIncomeLossDerivativesQualifyingAsHedgesTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParentAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of tax expense (benefit) before reclassification adjustments of unrealized holding gain (loss) on available-for-sale securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=66004124&loc=d3e640-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (e,f) -URI http://asc.fasb.org/extlink&oid=66004124&loc=SL7669646-108580 + Details
Name: | us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateContractMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_CommodityContractMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |