Debt Instrument, Number of Series Issued

No definition available.

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- Definition

Debt Instrument, Remaining Maturity Term

+ References

No definition available.

+ Details

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- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

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Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: 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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ 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 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Amount of debt issuance costs (for example, but not limited to, legal, accounting, broker, and regulatory fees).

+ References

No definition available.

+ Details

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Balance Type:	debit
Period Type:	duration

- Definition

Period the derivative contract is outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 19, 20, 22
-Article 5

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ 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(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

The cash inflow provided by derivative instruments during the period, which are classified as financing activities, excluding those designated as hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 23
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3505-108585

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Balance Type:	debit
Period Type:	duration

- Definition

The net cash inflow or cash outflow from issuing (borrowing) and repaying commercial paper.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 9
-Subparagraph c
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3098-108585

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Balance Type:	debit
Period Type:	duration

- Definition

The cash inflow from a borrowing having initial term of repayment of more than three months but less than one year or one operating cycle (if the normal cycle is more than one year).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Financing Activities
-URI http://asc.fasb.org/extlink&oid=6513228

+ Details

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Balance Type:	debit
Period Type:	duration

- Definition

Indebtedness divided by net capital.

+ References

No definition available.

+ Details

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- Definition

The cash outflow for a long-term debt where the holder has highest claim on the entity's asset in case of bankruptcy or liquidation during the period.

+ 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

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-Publisher FASB
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-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3291-108585

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- Details

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- Details

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- Details

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- Details

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Debt and Credit Facilities - Narrative (Details)				12 Months Ended
Debt and Credit Facilities - Narrative (Details)	Nov. 18, 2015 USD ($)	Jan. 23, 2015 USD ($)	Jan. 15, 2015 USD ($)	Dec. 31, 2015 USD ($) series	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Oct. 30, 2015 USD ($)
Debt Instrument [Line Items]
Net issuances (repayments) of commercial paper				$ 165,000,000	$ (4,000,000)	$ 206,000,000
Commercial paper issuance borrowings				0
Repayments of senior notes				$ 200,000,000	$ 0	$ 225,000,000
Debt-to-capitalization ratio (as a percent)				0.7
Nicor Gas [Member]
Debt Instrument [Line Items]
Debt-to-capitalization ratio (as a percent)				0.56	0.62
AGL Capital and Nicor Gas [Member]
Debt Instrument [Line Items]
Credit facility expiring fourth quarter of 2017							$ 75,000,000
AGL Capital [Member]
Debt Instrument [Line Items]
Borrowings outstanding under credit facility				$ 0	$ 0
Nicor Gas [Member]
Debt Instrument [Line Items]
Borrowings outstanding under credit facility				0	$ 0
Interest Rate Swap [Member]
Debt Instrument [Line Items]
Derivative, notional amount	$ 200,000,000	$ 800,000,000
Proceeds from derivative instrument, financing activities				248,000,000
Debt issuance cost				$ 2,000,000
Interest Rate Swap [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Derivative, term of contract		10 years
Interest Rate Swap [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Derivative, term of contract		30 years
Senior Notes [Member]
Debt Instrument [Line Items]
Repayments of senior notes			$ 200,000,000
Commercial Paper [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Debt instrument, term				1 day
Remaining maturity term				4 days
Commercial Paper [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Debt instrument, term				63 days
Remaining maturity term				43 days
Senior 3.875% 10 year Note, Due November 2025 [Member] \| Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, term	10 years
Issuance of long-term debt	$ 250,000,000
Stated interest rate (as a percent)	3.875%
Gas Facility Revenue Bonds [Member]
Debt Instrument [Line Items]
Number of series issued (in series) \| series				5