Represents the percentage of discretionary funds used to resolve natural gas imbalance

No definition available.

Name:	gas_DiscretionaryFundsUsedForNaturalGasImbalancePercentage
Namespace Prefix:	gas_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represent the term of the imbalance related to natural gas market.

+ References

No definition available.

+ Details

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Namespace Prefix:	gas_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	gas_Note11CommitmentsGuaranteesandContingenciesDetailsLineItems
Namespace Prefix:	gas_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Pertains to the percent a company is obligated for the natural gas imbalance.

+ References

No definition available.

+ Details

Name:	gas_PercentOfNaturalGasImbalanceObligated
Namespace Prefix:	gas_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of debt premium that was originally recognized at the issuance of the instrument that has yet to be amortized.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 1A

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of the gas imbalance in terms of units of measure (Mcf) as of the end of the period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 10

-Section S99

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=27015464&loc=d3e512571-122862

+ Details

Name:	us-gaap_GasImbalanceVolumeAmount
Namespace Prefix:	us-gaap_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	instant

- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 460

-SubTopic 10

-Section 50

-Paragraph 4

-Subparagraph (b)(1)

-URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Carrying value as of the balance sheet date of [accrued] interest payable on all forms of debt, including trade payables, that has been incurred and is unpaid. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 20

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Current Liabilities

-URI http://asc.fasb.org/extlink&oid=6509677

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section 45

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6935-107765

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.20)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section 45

-Paragraph 9

-URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e7018-107765

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of addition (reduction) to the amount at which a liability could be incurred (settled) in a current transaction between willing parties.

+ References

No definition available.

+ Details

Name:	us-gaap_LiabilitiesFairValueAdjustment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of long-term debt, after unamortized discount or premium, scheduled to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19,20)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 20

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19

-Article 5

+ Details

Name:	us-gaap_LongTermDebtCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The minimum amount the entity agreed to spend under the long-term purchase commitment.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Volume that must be purchased within the specified time period of the agreement to avoid penalties or unfavorable effects on pricing, among other adverse consequences.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermPurchaseCommitmentMinimumVolumeRequired
Namespace Prefix:	us-gaap_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of loss contingency liability.

+ References

+ Details

Name:	us-gaap_LossContingencyAccrualAtCarryingValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of increase (decrease) in loss contingency liability.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of damages awarded to the plaintiff in the legal matter.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 4

-Subparagraph (a)

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 9

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349

+ Details

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

- Definition

Amount of damages paid to the plaintiff in the legal matter.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The value (monetary amount) of the award the plaintiff seeks in the legal matter.

+ References

+ Details

Name:	us-gaap_LossContingencyDamagesSoughtValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Reflects the estimated amount of loss from the specified contingency as of the balance sheet date.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 4

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 9

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349

+ Details

Name:	us-gaap_LossContingencyEstimateOfPossibleLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	dei_LegalEntityAxis=gas_AtlantaGasLightMember
Namespace Prefix:
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Balance Type:
Period Type:

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=gas_IAGOMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=gas_IllinoisCommissionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=gas_FirstMortgageBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GuaranteeObligationsByNatureAxis=us-gaap_FinancialGuaranteeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_NatureOfExpenseAxis=gas_NaturalGasRemainderMember
Namespace Prefix:
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Period Type:

Note 11 - Commitments, Guarantees and Contingencies (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended		12 Months Ended	0 Months Ended		1 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended
	Dec. 31, 2014 ft3	Sep. 30, 2014 ft3	Dec. 31, 2014 ft3	Aug. 07, 2014	Jun. 07, 2013	Nov. 30, 2012	Dec. 31, 2012	Jun. 30, 2014	Dec. 31, 2009	Dec. 31, 2013	Feb. 28, 2012
Note 11 - Commitments, Guarantees and Contingencies (Details) [Line Items]
Debt Instrument, Unamortized Premium	$ 16		$ 16
Long-term Debt, Current Maturities	200us-gaap_LongTermDebtCurrent		200us-gaap_LongTermDebtCurrent
Long-term Purchase Commitment, Minimum Volume Required (in Cubic Feet)			51,000,000,000
Long-term Purchase Commitment, Amount			142
Interest Payable, Current	53us-gaap_InterestPayableCurrent		53us-gaap_InterestPayableCurrent							52us-gaap_InterestPayableCurrent
Natural Gas Imbalance Resolution Period		5 years
Gas Balancing Volume Amount (in Cubic Feet)	1,150,000,000us-gaap_GasImbalanceVolumeAmount	4,600,000,000us-gaap_GasImbalanceVolumeAmount	1,150,000,000us-gaap_GasImbalanceVolumeAmount
Discretionary Funds Used For Natural Gas Imbalance, Percentage	25.00%
Financial Guarantee [Member] \| Tropic Equipment Leasing Inc.[Member]
Note 11 - Commitments, Guarantees and Contingencies (Details) [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	1		1
First Mortgage Bonds [Member]
Note 11 - Commitments, Guarantees and Contingencies (Details) [Line Items]
Liabilities, Fair Value Adjustment			75
Interest Rate Swap [Member]
Note 11 - Commitments, Guarantees and Contingencies (Details) [Line Items]
Liabilities, Fair Value Adjustment			5
Natural Gas Remainder [Member]
Note 11 - Commitments, Guarantees and Contingencies (Details) [Line Items]
Gas Balancing Volume Amount (in Cubic Feet)	2,300,000,000		2,300,000,000
Atlanta Gas Light [Member]
Note 11 - Commitments, Guarantees and Contingencies (Details) [Line Items]
Gas Balancing Volume Amount (in Cubic Feet)	1,150,000,000		1,150,000,000
Percent of NaturalGas Imbalance Obligated	25.00%
Illinois Commission [Member]
Note 11 - Commitments, Guarantees and Contingencies (Details) [Line Items]
Loss Contingency, Damages Sought, Value				18					85
Loss Contingency, Estimate of Possible Loss											64
Loss Contingency, Damages Awarded, Value					72	72
Loss Contingency Accrual, Period Increase (Decrease)							8
Loss Contingency Accrual							72
Loss Contingency, Damages Paid, Value								43
CUB [Member]
Note 11 - Commitments, Guarantees and Contingencies (Details) [Line Items]
Loss Contingency, Damages Sought, Value				22					305
IAGO [Member]
Note 11 - Commitments, Guarantees and Contingencies (Details) [Line Items]
Loss Contingency, Damages Sought, Value									$ 255