The stated principal amount of the debt instrument at time of issuance, which may vary from the carrying amount because of unamortized premium or discount.

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- Definition

Reflects the term of the credit facility, which may be presented in a variety of ways (years, months, days).

+ Details

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- Definition

Date when the debt instrument is scheduled to be fully repaid

+ Details

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- Definition

Identification of the contractual issuer of the short-term debt instrument.

+ Details

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- Definition

Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.13(2))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 9

-Section 03

-Paragraph 13

-Subparagraph 2

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-Name Regulation S-X (SX)

-Number 210

-Article 7

-Section 03

-Paragraph 16

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-Name Regulation S-X (SX)

-Number 210

-Article 5

-Section 02

-Paragraph 19

-Subparagraph a(3)

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(a)(3))

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

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- Definition

Interest rate stated in the contractual debt agreement.

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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-Name Accounting Standards Codification

-Topic 210

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-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

Date when the debt instrument was issued, which may be presented in a variety of ways (year, month and year, day, month and year, quarter, etc.).

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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

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-Name Accounting Standards Codification

-Topic 210

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-Paragraph 1

-Subparagraph (SX 210.5-02.22(a))

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Identification of the contractual issuer of the debt instrument (for example, a subsidiary of the reporting entity).

+ References

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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.

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- Definition

Reflects when the credit facility terminates, which may be presented in a variety of ways (year, month and year, day, month and year, quarter).

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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Reflects when the credit facility first became available, which may be presented in a variety of ways (year, month and year, day, month and year, quarter).

+ References

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- 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

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- Definition

Aggregate notional amount of interest rate derivatives, which relates to the currency amount specified in the interest rate derivative instruments.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1B

-URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5580258-113959

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-Subparagraph (d)

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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.

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SHORT-TERM AND LONG-TERM DEBT (Details) (USD $) In Millions, unless otherwise specified				9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2010	Sep. 30, 2011 First Mortgage Bonds [Member] First Mortgage Bonds - 6.625% [Member]	Sep. 30, 2011 First Mortgage Bonds [Member] First Mortgage Bonds - 2.86% [Member]	Sep. 30, 2011 Revolving Credit Facility [Member] Revolving Credit Facility Expiring April 2012 [Member]	Sep. 30, 2011 Revolving Credit Facility [Member] Revolving Credit Facility Expiring April 2011 [Member]	Sep. 30, 2011 Revolving Credit Facility [Member] Revolving Credit Facility Expiring April 2013 [Member]	Sep. 30, 2011 Revolving Credit Facility [Member] Revolving Credit Facility Expiring September 2010 [Member]
Long-term Debt Instruments [Line Items]
Long Term Debt Issuer				Nicor Gas	Nicor Gas
Face amount of First Mortgage Bonds				$ 75	$ 75
Interest rate on First Mortgage Bonds (in hundredths)				6.625%	2.86%
Issuance date of First Mortgage Bonds					February 2011
Maturity date of First Mortgage Bonds				February 2011	2016
Short-term Debt [Line Items]
Short Term Debt Issuer						Nicor Gas	Nicor Gas	Nicor and Nicor Gas	Nicor and Nicor Gas
Maximum borrowing capacity on revolving credit facility						400	400	600	600
Initiation date of revolving credit facility						April 2011		April 2010
Expiration date of revolving credit facility						April 2012	April 2011	April 2013	September 2010
Term of revolving credit facility						364-day	364-day	three-year	five-year
Commercial paper borrowings outstanding	312.6	425.0	373.3
Notional amount of forward-starting interest rate swaps	$ 90	$ 90	$ 90