Represents Employee Stock Purchase Plan, maximum contributions per employee.

No definition available.

Name:	gas_EmployeeStockPurchasePlanMaximumContributionsPerEmployee
Namespace Prefix:	gas_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum payout on performance share unit awards as awarded by a company to their employees as a form of incentive compensation.

+ References

No definition available.

+ Details

Name:	gas_MaximumAggregatePayout
Namespace Prefix:	gas_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	gas_Note7StockbasedCompensationDetailsLineItems
Namespace Prefix:	gas_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate carrying value as of the balance sheet date of the liabilities for stock option plans and other equity-based compensation arrangements.

+ References

No definition available.

+ Details

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

- Definition

Unrecognized cost of unvested options awarded to employees as compensation.

+ References

No definition available.

+ Details

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

- Definition

The cash inflow associated with the amount received from holders exercising their stock options. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (j)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 14

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3255-108585

+ Details

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

- Definition

Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of vesting of share-based compensation awards.

+ References

+ Details

Name:	us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Discount rate from fair value on purchase date that participants pay for shares.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPricePurchaseDate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)(iii)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)(iii)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)(2)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of additional shares authorized for issuance under an established share-based compensation plan.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Gross number of share options (or share units) granted during the period.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Purchase price of common stock expressed as a percentage of its fair value.

+ References

No definition available.

+ Details

Name:	us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The floor of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The ceiling of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_AwardTypeAxis=gas_StockAwardsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_EmployeeStockOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_PerformanceSharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockUnitsRSUMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=gas_EmployeeStockPurchasePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=gas_LongTermIncentivePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=gas_OmnibusPerformanceIncentivePlanMember
Namespace Prefix:
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Period Type:

Note 7 - Stock-based Compensation (Details) (USD $)	12 Months Ended
Note 7 - Stock-based Compensation (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Note 7 - Stock-based Compensation (Details) [Line Items]
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options (in Dollars)	$ 0	$ 0
Proceeds from Stock Options Exercised (in Dollars)	9,000,000	21,000,000
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Lower Range Limit (in Dollars per share)	$ 31.09
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Upper Range Limit (in Dollars per share)	$ 43.54
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	0	0	0
Employee Stock Option [Member]
Note 7 - Stock-based Compensation (Details) [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period	10 years
Restricted Stock Units (RSUs) [Member]
Note 7 - Stock-based Compensation (Details) [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	44,272
Share-based Compensation Arrangement by Share-based Payment Award, Award Requisite Service Period	3 years
Performance Shares [Member] \| Granted in 2012 [Member]
Note 7 - Stock-based Compensation (Details) [Line Items]
Deferred Compensation Share-based Arrangements, Liability, Current and Noncurrent (in Dollars)	8,000,000
Maximum Aggregate Payout (in Dollars)	20,000,000
Stock Awards [Member] \| Non-Employee Directors [Member]
Note 7 - Stock-based Compensation (Details) [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	21,903
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage	100.00%
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value (in Dollars per share)	$ 52.97
Omnibus Performance Incentive Plan [Member]
Note 7 - Stock-based Compensation (Details) [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	587,292
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized	3,962,335
Long-term Incentive Plan [Member] \| Nicor Gas [Member]
Note 7 - Stock-based Compensation (Details) [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	1,551,040
Long-term Incentive Plan [Member]
Note 7 - Stock-based Compensation (Details) [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	436,400
Employee Stock Purchase Plan [Member]
Note 7 - Stock-based Compensation (Details) [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	422,564
Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent	85.00%
Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Purchase Date	15.00%
Employee Stock Purchase Plan, Maximum Contributions Per Employee (in Dollars)	$ 25,000