Regulated Operations (Details) $ in Millions	1 Months Ended		3 Months Ended
	Oct. 31, 2015 USD ($)	Jul. 31, 2014 USD ($) installment	Jun. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) site	Feb. 28, 2015 USD ($)	Dec. 31, 2014 USD ($)
Regulated Operations [Line Items]
Regulatory assets - current				$ 105	$ 64		$ 83
Number of former operating sites | site					46
Insurance settlement receivable		$ 77
Insurance settlement receivable, number of installments | installment		2
Proceeds from insurance settlement			$ 32	$ 45
Recoverable ERC Assets [Member]
Regulated Operations [Line Items]
Regulatory assets - current					$ 28
Nicor Gas [Member]
Regulated Operations [Line Items]
Number of former operating sites | site					26
Number of sites with completed cleanups | site					2
Number of sites with cleanup responsibility | site					23
Cleanup costs allocated (as a percent)					52.00%
True-up Recovery [Member] | Atlanta Gas Light [Member]
Regulated Operations [Line Items]
Unrecovered program revenue to be recovered						$ 178
True-up Recovery [Member] | Subsequent Event [Member] | Atlanta Gas Light [Member]
Regulated Operations [Line Items]
Allowable amount of costs incurred to be recovered	$ 144