Commitments, Guarantees and Contingencies (Details)	1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 31, 2015 USD ($) $ / item	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Feb. 28, 2015 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Guarantor Obligations, Current Carrying Value		$ 0		$ 0
Gain Contingencies [Line Items]
Purchase obligation period				10 years
Interest expense		42,000,000	$ 44,000,000	$ 128,000,000	$ 135,000,000
Operation and maintenance		204,000,000	$ 193,000,000	662,000,000	$ 693,000,000
Nicor Gas [Member]
Gain Contingencies [Line Items]
Long-term purchase obligations		978,000,000		978,000,000
True-up Recovery [Member] | Atlanta Gas Light [Member]
Gain Contingencies [Line Items]
Unrecovered program revenue to be recovered						$ 178,000,000
Interest expense		1,000,000		1,000,000
Operation and maintenance		6,000,000		5,000,000
Capitalized mitigation costs		$ 28,000,000		$ 28,000,000
Subsequent Event [Member] | Atlanta Gas Light [Member]
Gain Contingencies [Line Items]
Cumulative PRP surcharge increase (in dollars per item) | $ / item	2.44
Subsequent Event [Member] | True-up Recovery [Member] | Atlanta Gas Light [Member]
Gain Contingencies [Line Items]
Allowable amount of costs incurred to be recovered	$ 144,000,000
PRP surcharge increase beginning Oct. 1, 2015 (in dollars per item) | $ / item	0.82
PRP surcharge increase beginning Oct. 1, 2016 (in dollars per item) | $ / item	0.81
PRP surcharge increase beginning Oct. 1, 2017 (in dollars per item) | $ / item	0.81
Subsequent Event [Member] | Favorable Regulatory Action [Member] | Atlanta Gas Light [Member]
Gain Contingencies [Line Items]
Final order for rate true up	$ 144,000,000