Derivative Instruments (Details) - Gains and Losses on Derivative Instruments - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Nicor Gas [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total volumes	$ (15)	$ (4)	$ (36)	$ 8
Elizabethtown Gas [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total volumes	$ (4)	$ (1)	$ (12)	$ 4