DERIVATIVE INSTRUMENTS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Volumes [Abstract]
Outstanding derivative contracts to hedge gas purchases for customer use (in Bcf)	30	48	30	48	49
Remaining term of outstanding derivative contracts to hedge natural gas purchases for customer use (in years)			three years	three years	three years
Outstanding derivative contracts related to Nicor Enerchange's and Nicor Gas' company use, short (long) (in Bcf)	4.5	3.0	4.5	3.0	3.9
Notional amount of forward-starting interest rate swaps	$ 90	$ 90	$ 90	$ 90	$ 90
Cash flow hedges [Abstract]
Pretax (gain) loss recognized in other comprehensive income (effective portion)	10.8	5.8	14.1	13.1
Time horizon of cash flow hedges of natural gas purchases for Nicor Gas company use and for utility-bill management products sold by Nicor's other energy ventures (in years)			two years	two years	two years
Pretax (gain) loss deferred in accumulated other comprehensive income for certain commodity derivatives designated as cash flow hedges	2.2	4.2	2.2	4.2	2.0
After tax (gain) loss deferred in accumulated other comprehensive income for certain commodity derivatives designated as cash flow hedges	1.3	2.5	1.3	2.5	1.2
Pretax (gain) loss deferred in accumulated other comprehensive income relating to forward-starting interest rate swaps	13.7	7.5	13.7	7.5	2.2
After tax (gain) loss deferred in accumulated other comprehensive income relating to forward-starting interest rate swaps	8.1	4.5	8.1	4.5	1.3
Derivatives not designated as hedges [Abstract]
Net gains (losses) relating to derivatives not designated as hedging instruments deferred as regulatory assets or liabilities	29.0	47.5	32.6	121.6
Credit-risk-related contingent features [Abstract]
Fair value of derivative liabilities for contracts with credit-risk-related contingent features	17	17	17	17	12
Amount that would be required to pay if the company were required to post the maximum contractually specified collateral or settle the liability	17	16	17	16	11
Derivative Instruments, Gain / Loss [Line Items]
Pretax (gain) loss reclassified from accumulated other comprehensive income into income (effective portion)	0.3	0.6	2.2	2.2
Pretax net gain (loss) recognized in income	4.2	5.0	(0.1)	2.9
Derivatives designated as Hedging Instruments [Member]
Assets [Abstract]
Carrying amount of derivative assets	0.5	0	0.5	0	1.4
Liabilities [Abstract]
Carrying amount of derivative liabilities	16.0	11.1	16.0	11.1	5.4
Derivatives designated as Hedging Instruments [Member] \| Derivative Instruments [Member]
Assets [Abstract]
Carrying amount of derivative assets	0.5	0	0.5	0	0.4
Liabilities [Abstract]
Carrying amount of derivative liabilities	15.9	3.4	15.9	3.4	2.2
Derivatives designated as Hedging Instruments [Member] \| Other - noncurrent asset [Member]
Assets [Abstract]
Carrying amount of derivative assets	0	0	0	0	1.0
Derivatives designated as Hedging Instruments [Member] \| Other - noncurrent liabilities
Liabilities [Abstract]
Carrying amount of derivative liabilities	0.1	7.7	0.1	7.7	3.2
Derivatives Not Designated as Hedging Instruments [Member] \| Operating Revenues Member
Derivative Instruments, Gain / Loss [Line Items]
Pretax net gain (loss) recognized in income	4.5	5.3	0.3	3.9
Derivatives Not Designated as Hedging Instruments [Member] \| Operating And Maintenance Member
Derivative Instruments, Gain / Loss [Line Items]
Pretax net gain (loss) recognized in income	(0.3)	(0.3)	(0.4)	(1.0)
Derivatives Not Designated as Hedging Instruments [Member]
Assets [Abstract]
Carrying amount of derivative assets	61.1	93.4	61.1	93.4	65.9
Liabilities [Abstract]
Carrying amount of derivative liabilities	71.4	136.0	71.4	136.0	96.8
Derivatives Not Designated as Hedging Instruments [Member] \| Derivative Instruments [Member]
Assets [Abstract]
Carrying amount of derivative assets	49.1	70.4	49.1	70.4	48.7
Liabilities [Abstract]
Carrying amount of derivative liabilities	64.4	112.5	64.4	112.5	80.7
Derivatives Not Designated as Hedging Instruments [Member] \| Other - noncurrent asset [Member]
Assets [Abstract]
Carrying amount of derivative assets	12.0	23.0	12.0	23.0	17.2
Derivatives Not Designated as Hedging Instruments [Member] \| Other - noncurrent liabilities
Liabilities [Abstract]
Carrying amount of derivative liabilities	7.0	23.5	7.0	23.5	16.1
Cash Flow Hedging [Member] \| Operating Revenues Member
Derivative Instruments, Gain / Loss [Line Items]
Pretax (gain) loss reclassified from accumulated other comprehensive income into income (effective portion)	0	0.1	1.4	1.1
Cash Flow Hedging [Member] \| Operating And Maintenance Member
Derivative Instruments, Gain / Loss [Line Items]
Pretax (gain) loss reclassified from accumulated other comprehensive income into income (effective portion)	$ 0.3	$ 0.5	$ 0.8	$ 1.1

Amount of pretax gains or losses deferred into accumulated other comprehensive income as of the balance sheet date related to certain commodity derivatives.

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Fair value of derivative asset, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

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The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

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Maximum length of time over which the entity is hedging its exposure to the variability in future cash flows for forecasted transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments.

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

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