CONTINGENCIES (Details) (USD $) In Millions, unless otherwise specified		12 Months Ended			6 Months Ended
	Jun. 30, 2011	Dec. 31, 2004 PBR Plan [Member]	Jun. 30, 2011 PBR Plan [Member]	Dec. 31, 2003 PBR Plan [Member]	Jun. 30, 2011 PBR Plan [Member] Illinois Commerce Commission [Member]	Jun. 30, 2011 PBR Plan [Member] Illinois Attorney General's Office [Member]	Jun. 30, 2011 PBR Plan [Member] Citizens Utility Board [Member]	Jun. 30, 2011 Mercury [Member]	Jun. 30, 2011 Manufactured Gas Plant Sites [Member]
PBR Plan Claims [Abstract]
Initial PBR reserve				$ 24.8
Loss contingency included in PBR reserve			4.1
2004 adjustment to previously recorded PBR reserve		1.8
Recorded PBR reserve			26.6
Unrecognized 2002 PBR claimed receivable			26.9
Amount of sanctions sought per the February 3, 2003 CUB motion							27.0
Unrecognized recovery sought by Nicor Gas			6.0
Unrecognized amount of interest included in recovery amount sought by Nicor Gas			1.6
Amount of customer refunds requested by parties					109	255	286
Litigation Relating to Proposed Merger [Abstract]
Proposed settlement amount	0.7
Mercury Loss Contingency Estimate [Abstract]
Reserve recorded for environmental matters								0.5
Manufactured Gas Plant Sites Loss Contingency Estimate [Abstract]
Percent of cleanup costs allocated to company for certain sites (in hundredths)									51.73%
Number of sites for which company has 51.73% of clean-up responsibility									24
Number of sites with no cleanup responsibility									14
Percent of costs allocated to Company for general remediation program costs that do not relate exclusively to particular sites (in hundredths)									50.00%
Reserve Recorded									$ 52.9