FAIR VALUE MEASUREMENTS (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Assets
Money market funds	$ 59.0	$ 112.8	$ 59.0	$ 112.8	$ 82.9
Commodity derivatives	48.0	58.4	48.0	58.4	66.3
Interest rate derivative					1.0
Total assets	107.0	171.2	107.0	171.2	150.2
Liabilities
Commodity derivatives	61.4	109.9	61.4	109.9	99.0
Interest rate derivative	5.0	4.5	5.0	4.5	3.2
Total Liabilities	66.4	114.4	66.4	114.4	102.2
Beginning of period	(3.8)	(7.6)	(3.0)	5.0
Net realized/unrealized gains (losses)
Included in regulatory assets and liabilities	2.2	0.5	5.1	(0.8)
Included in net income	(0.1)	(0.5)	(0.5)	(6.8)
Settlements	2.0		0.5
Settlements, net of purchases		3.1		3.9
Transfers into Level 3	0	2.8	1.5	2.8
Transfers out of Level 3	7.2	8.8	3.9	3.0
End of period	7.5	7.1	7.5	7.1
Net unrealized gains (losses) included in net income relating to derivatives still held and classified as level 3	0.6	(0.2)	0.5	0.9
Balance Sheet Classification of Margin Assets [Abstract]
Margin accounts - derivative instruments	33.8	52.6	33.8	52.6	50.9
Other - noncurrent	3.8	13.5	3.8	13.5	8.1
Balance Sheet Classification of Margin Liabilities [Abstract]
Other - current	0.3	1.9	0.3	1.9	0
Other - noncurrent	0.6	3.1	0.6	3.1	0
Principal balance of Nicor Gas' First Mortgage Bonds outstanding	500	500	500	500	500
Fair value of Nicor Gas' First Mortgage Bonds outstanding	566	561	566	561	554
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets (Level 1) [Member]
Assets
Money market funds	59.0	112.8	59.0	112.8	82.9
Commodity derivatives	11.4	20.7	11.4	20.7	21.3
Total assets	70.4	133.5	70.4	133.5	104.2
Liabilities
Commodity derivatives	36.5	56.4	36.5	56.4	55.4
Total Liabilities	36.5	56.4	36.5	56.4	55.4
Fair Value, Measurements, Recurring [Member] | Significant observable inputs (Level 2) [Member]
Assets
Commodity derivatives	27.4	27.5	27.4	27.5	35.7
Interest rate derivative					1.0
Total assets	27.4	27.5	27.4	27.5	36.7
Liabilities
Commodity derivatives	23.2	50.4	23.2	50.4	31.3
Interest rate derivative	5.0	4.5	5.0	4.5	3.2
Total Liabilities	28.2	54.9	28.2	54.9	34.5
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member]
Assets
Commodity derivatives	9.2	10.2	9.2	10.2	9.3
Total assets	9.2	10.2	9.2	10.2	9.3
Liabilities
Commodity derivatives	1.7	3.1	1.7	3.1	12.3
Total Liabilities	$ 1.7	$ 3.1	$ 1.7	$ 3.1	$ 12.3