FAIR VALUE MEASUREMENTS (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Assets
Money market funds	$ 59.0	$ 128.8	$ 59.0	$ 128.8	$ 82.9
Commodity derivatives	61.6	93.4	61.6	93.4	66.3
Interest rate derivative					1.0
Total assets	120.6	222.2	120.6	222.2	150.2
Liabilities
Commodity derivatives	73.7	139.6	73.7	139.6	99.0
Interest rate derivative	13.7	7.5	13.7	7.5	3.2
Total Liabilities	87.4	147.1	87.4	147.1	102.2
Beginning of period	7.5	7.1	(3.0)	5.0
Net realized/unrealized gains (losses)
Included in regulatory assets and liabilities	(3.7)	(0.9)	1.4	(1.7)
Included in net income	2.1	4.8	1.6	(2.0)
Settlements	(1.9)		(1.4)
Settlements, net of purchases		0.6		4.5
Transfers into Level 3	0.6	(13.6)	2.1	(10.8)
Transfers out of Level 3	0.1	(4.9)	4.0	(1.9)
End of period	4.7	(6.9)	4.7	(6.9)
Net unrealized gains (losses) included in net income relating to derivatives still held and classified as level 3	1.2	1.7	1.3	(4.0)
Balance Sheet Classification of Margin Assets [Abstract]
Margin accounts - derivative instruments	27.5	58.8	27.5	58.8	50.9
Other - noncurrent	0.6	9.5	0.6	9.5	8.1
Balance Sheet Classification of Margin Liabilities [Abstract]
Other - current	2.2	2.2	2.2	2.2	0
Other - noncurrent	0.4	4.5	0.4	4.5	0
Principal balance of Nicor Gas' First Mortgage Bonds outstanding	500	500	500	500	500
Fair value of Nicor Gas' First Mortgage Bonds outstanding	621	575	621	575	554
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets (Level 1) [Member]
Assets
Money market funds	59.0	128.8	59.0	128.8	82.9
Commodity derivatives	22.0	42.0	22.0	42.0	21.3
Total assets	81.0	170.8	81.0	170.8	104.2
Liabilities
Commodity derivatives	47.9	83.1	47.9	83.1	55.4
Total Liabilities	47.9	83.1	47.9	83.1	55.4
Fair Value, Measurements, Recurring [Member] | Significant observable inputs (Level 2) [Member]
Assets
Commodity derivatives	32.4	42.4	32.4	42.4	35.7
Interest rate derivative					1.0
Total assets	32.4	42.4	32.4	42.4	36.7
Liabilities
Commodity derivatives	23.3	40.6	23.3	40.6	31.3
Interest rate derivative	13.7	7.5	13.7	7.5	3.2
Total Liabilities	37.0	48.1	37.0	48.1	34.5
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member]
Assets
Commodity derivatives	7.2	9.0	7.2	9.0	9.3
Total assets	7.2	9.0	7.2	9.0	9.3
Liabilities
Commodity derivatives	2.5	15.9	2.5	15.9	12.3
Total Liabilities	$ 2.5	$ 15.9	$ 2.5	$ 15.9	$ 12.3