Regulated Operations - Narrative (Details) $ in Millions	1 Months Ended						3 Months Ended		12 Months Ended
	Oct. 31, 2015 USD ($)	Sep. 30, 2015 USD ($) station mi	Jul. 31, 2014 USD ($) installment	May. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Aug. 31, 2013 USD ($)	Jun. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2015 USD ($) site mi	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($) mi	Dec. 31, 2012 USD ($)	Nov. 30, 2015 USD ($)	Feb. 28, 2015 USD ($)	May. 31, 2011 USD ($)
Regulated Operations [Line Items]
Regulatory assets									$ 68	$ 83
Accrued/deferred natural gas costs									$ 6	67	$ (2)
Number of former sites | site									46
Insurance settlements receivable			$ 77
Insurance settlements receivable, number of installments | installment			2
Proceeds from insurance settlement							$ 32	$ 45
Nicor Gas [Member]
Regulated Operations [Line Items]
Number of former sites | site									26
Number of sites with completed cleanups (in sites) | site									2
Number of sites with cleanup responsibility (in sites) | site									23
Cleanup costs allocated (as a percent)									52.00%
Infrastructure investment, annual customer rate increase, not to exceed (as a percent)											4.00%
Florida City Gas [Member]
Regulated Operations [Line Items]
Infrastructure relocation and enhancement		$ 10
Infrastructure relocation period		10 years
Recoverable Pension and Retiree Welfare Costs [Member]
Regulated Operations [Line Items]
Regulatory assets									$ 12	12
Recoverable Pension and Retiree Welfare Costs [Member] | Nicor Gas [Member]
Regulated Operations [Line Items]
Cost recovery period									10 years
Recoverable ERC Assets [Member]
Regulated Operations [Line Items]
Accrued/deferred natural gas costs									$ 40	51	$ 24
Recoverable Regulatory Infrastructure Program Costs [Member] | Nicor Gas [Member]
Regulated Operations [Line Items]
Cost recovery period									10 years
Recoverable Environmental Remediation Costs [Member]
Regulated Operations [Line Items]
Regulatory assets									$ 31	49
Regulatory Asset Off Balance Sheet [Member]
Regulated Operations [Line Items]
Weighted average cost of capital (as a percent)											4.27%
Seasonal Rates [Member]
Regulated Operations [Line Items]
Regulatory assets									$ 10	10
Seasonal Rates [Member] | Atlanta Gas Light [Member]
Regulated Operations [Line Items]
Cost recovery period									1 year
Minimum [Member] | Recoverable Pension and Retiree Welfare Costs [Member]
Regulated Operations [Line Items]
Cost recovery period									8 years
Maximum [Member] | Recoverable Pension and Retiree Welfare Costs [Member]
Regulated Operations [Line Items]
Cost recovery period									17 years
STRIDE [Member] | Atlanta Gas Light [Member]
Regulated Operations [Line Items]
Program duration period					4 years
Integrated Vintage Plastic Replacement Program [Member] | Atlanta Gas Light [Member]
Regulated Operations [Line Items]
Approved infrastructure replacement program					$ 275						$ 275
Program duration period											4 years
Natural gas pipeline length, considered for replacement (in miles) | mi									3,300
Period for replacement, lower range									15 years
Period for replacement, upper range									20 years
Natural gas pipeline length, approved for replacement (in miles) | mi											756
Safety, Modernization and Reliability Tariff (SMART) Plan [Member] | Elizabethtown Gas [Member]
Regulated Operations [Line Items]
Natural gas pipeline length, considered for replacement (in miles) | mi		630
Infrastructure replacement program, proposed for approval		$ 1,100
Number of regulator stations, considered for replacement | station		240
Rider surcharge, recovery period									10 years
Enhanced Infrastructure Program [Member] | Elizabethtown Gas [Member]
Regulated Operations [Line Items]
Approved infrastructure replacement program						$ 115									$ 40
Weighted average cost of capital (as a percent)											6.65%
Accelerated Infrastructure Replacement Program [Member] | Elizabethtown Gas [Member]
Regulated Operations [Line Items]
Program duration period						4 years
Natural Gas Distribution Utility Reinforcement Effort [Member] | Elizabethtown Gas [Member]
Regulated Operations [Line Items]
Infrastructure replacement program										$ 15
SAVE Program[Member] | Virginia Natural Gas [Member]
Regulated Operations [Line Items]
Approved infrastructure replacement program												$ 25
Program duration period												5 years
Funding in 2016													$ 30
Funding in 2017													35
Funding in 2018													35
Funding in 2019													35
Funding in 2020													35
Funding in 2021													$ 35
SAVE Program[Member] | Maximum [Member] | Virginia Natural Gas [Member]
Regulated Operations [Line Items]
Approved infrastructure replacement program												$ 105
Energy Smart Plan [Member] | Nicor Gas [Member]
Regulated Operations [Line Items]
Approved infrastructure replacement program				$ 93
Program duration period				3 years
Regulatory Income Tax Liability [Member]
Regulated Operations [Line Items]
Regulatory noncurrent liability, amortization period									30 years
i SRP Projects [Member] | STRIDE [Member] | Atlanta Gas Light [Member]
Regulated Operations [Line Items]
Approved infrastructure replacement program					445						$ 445
i CGP Projects [Member] | STRIDE [Member] | Atlanta Gas Light [Member]
Regulated Operations [Line Items]
Approved infrastructure replacement program					$ 91						$ 91
True Up Recovery [Member] | Atlanta Gas Light [Member]
Regulated Operations [Line Items]
Allowable cost recovery	$ 144
Unrecovered program revenue to be recovered														$ 178
Illinois [Member]
Regulated Operations [Line Items]
Number of former sites | site									26
Illinois [Member] | Nicor Gas [Member]
Regulated Operations [Line Items]
Number of former sites | site									26