Note 9 - Commitments, Guarantees and Contingencies (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			12 Months Ended		1 Months Ended	3 Months Ended	12 Months Ended			12 Months Ended
	Dec. 31, 2011	Dec. 31, 2004	Dec. 21, 2012 ft3	Dec. 31, 2011 Interest Due [Member]	Dec. 31, 2003 Adverse Consequences To Ratepayers [Member]	Nov. 30, 2012 Illinois Commission [Member]	Sep. 30, 2013 Illinois Commission [Member]	Dec. 31, 2011 Illinois Commission [Member]	Jun. 07, 2013 Illinois Commission [Member]	Feb. 01, 2012 Illinois Commission [Member]	Dec. 31, 2011 IAGO [Member]	Dec. 31, 2011 CUB [Member]
Note 9 - Commitments, Guarantees and Contingencies (Details) [Line Items]
Gas Balancing Volume Amount (in Cubic Feet)			4,800,000,000
Prior Period Reclassification Adjustment					$ 25
Loss Contingency, Estimate of Possible Loss		27			4				64	64
Adjustment To Previously Recorded Pbr Reserve		2
Loss Contingency, Damages Sought, Value	6			2				85			255	305
Loss Contingency, Damages Awarded, Value						72
Loss Contingency Accrual, Period Increase (Decrease)						8
Loss Contingency Accrual						72
Loss Contingency, Damages Paid, Value							$ 5