Note 5 - Derivative Instruments (Detail) - Gains and Losses on Derivative Instruments (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Sep. 30, 2011
Nicor Gas [Member] | Three Months Ended [Member]
RealizedLossesHedgingNaturalGasCosts	$ 12
Nicor Gas [Member] | Nine Months Ended [Member]
RealizedLossesHedgingNaturalGasCosts	60
Elizabethtown Gas [Member] | Three Months Ended [Member]
RealizedLossesHedgingNaturalGasCosts	6	6
Elizabethtown Gas [Member] | Nine Months Ended [Member]
RealizedLossesHedgingNaturalGasCosts	$ 23	$ 19