Note 5 - Derivative Instruments (Detail) - Gains and Losses on Derivative Instruments (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Jun. 30, 2011
Nicor Gas [Member] | Three Months Ended [Member]
Accrued Natural Gas Costs	$ 25
Nicor Gas [Member] | Six Months Ended [Member]
Accrued Natural Gas Costs	26
Elizabethtown Gas [Member] | Three Months Ended [Member]
Accrued Natural Gas Costs	7	5
Elizabethtown Gas [Member] | Six Months Ended [Member]
Accrued Natural Gas Costs	$ 16	$ 13