Significant Accounting Policies and Methods of Application - Narrative (Details)				1 Months Ended	3 Months Ended		12 Months Ended
	Sep. 01, 2014 USD ($)	Apr. 11, 2014 USD ($) mi	May. 01, 2013 USD ($)	Oct. 31, 2013	Sep. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) counterparty customer Bcf	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Accounting Policies [Line Items]
Inventory, net							$ 651,000,000	$ 716,000,000
Reduction in depreciation expense								51,000,000	$ 19,000,000
Regulatory liabilities - long-term							1,611,000,000	1,601,000,000
Goodwill							1,813,000,000	1,827,000,000
Goodwill impairment							$ 14,000,000	$ 0	0
Goodwill, impairment testing, fair value in excess of carrying value (less than) (as a percent)							5.00%	5.00%
Goodwill, impairment loss, net of tax							$ 9,000,000
Indefinite-lived intangible assets							0
Impairment of long-lived assets							0
Amortization of intangible assets							18,000,000	$ 20,000,000	18,000,000
Excise taxes collected							103,000,000	133,000,000	112,000,000
Disposition of assets							0	230,000,000	12,000,000
Gain on disposition of assets							$ 0	$ 2,000,000	$ 11,000,000
Atlanta Gas Light [Member]
Accounting Policies [Line Items]
Concentration risk, number of customers (in customers) | customer							14
Depreciation rate (as a percent)							2.40%	2.30%	2.60%
Nicor Gas [Member]
Accounting Policies [Line Items]
LIFO inventory amount							$ 145,000,000
Inventory, net							201,000,000
Excess of replacement or current costs over stated LIFO value							$ 56,000,000
Depreciation rate (as a percent)				3.07%			3.10%	3.10%	3.10%
Tropical Shipping [Member]
Accounting Policies [Line Items]
Goodwill, impairment loss, net of tax						$ 19,000,000
Income tax expense					$ 29,000,000	$ 31,000,000		$ 61,000,000	$ 3,000,000
Repatriation of earnings								86,000,000
Compass Energy [Member]
Accounting Policies [Line Items]
Disposition of assets			$ 12,000,000
Gain on disposition of assets			11,000,000
Gain disposition of assets, net of tax			$ 5,000,000
Contingent cash consideration, up to									8,000,000
Guaranteed minimum receipt					4,000,000				3,000,000
Remaining contingent consideration									$ 5,000,000
Contingent consideration, earn out period									5 years
Former gain contingency, recognized in current period					3,000,000
Integrity Tests Performed at Storage Facilities [Member]
Accounting Policies [Line Items]
Inventory write-down								10,000,000
Distributed Earnings Of Foreign Subsidiaries [Member]
Accounting Policies [Line Items]
Repatriation of earnings					$ 86,000,000
Distributed Earnings Of Foreign Subsidiaries [Member] | Tropical Shipping [Member]
Accounting Policies [Line Items]
Income tax expense	$ 60,000,000							60,000,000
Sawgrass Storage [Member]
Accounting Policies [Line Items]
Impairment of equity method investment									$ 8,000,000
Dalton Pipeline Arrangement 1 [Member]
Accounting Policies [Line Items]
Percentage of voting interests acquired (as a percent)		50.00%
Natural gas pipeline length (in miles) | mi		106
Undivided ownership interest to be leased (as a percent)		50.00%					50.00%
Future minimum payments receivable		$ 26,000,000
Operating leases, term of contract		25 years
Construction work in process							$ 33,000,000
Wholesale Services [Member]
Accounting Policies [Line Items]
Natural gas volume previously inaccessible (In Bcf) | Bcf							5
Wholesale Services [Member] | Accounts Receivable [Member] | Credit Concentration Risk [Member]
Accounting Policies [Line Items]
Number of top counterparties (in counterparties) | counterparty							20
Concentration risk (as a percent)							53.00%
Accounts receivable, net							$ 196,000,000
Midstream Operations [Member]
Accounting Policies [Line Items]
Goodwill							0	14,000,000
Goodwill impairment							14,000,000
Retail Operations [Member]
Accounting Policies [Line Items]
Goodwill							173,000,000	173,000,000
Goodwill impairment							$ 0
Utility-bill management contracts period							12 months
Storage Facilities [Member] | Asset Retirement Obligation Costs [Member]
Accounting Policies [Line Items]
Regulatory liabilities - long-term							$ 3,000,000	3,000,000
Included in Operating Expenses [Member]
Accounting Policies [Line Items]
Excise taxes collected							101,000,000	130,000,000	$ 110,000,000
New Accounting Pronouncement, Early Adoption, Effect [Member] | Long-term Debt [Member]
Accounting Policies [Line Items]
Debt issuance cost							20,000,000	21,000,000
New Accounting Pronouncement, Early Adoption, Effect [Member] | Other Assets [Member]
Accounting Policies [Line Items]
Debt issuance cost							$ (20,000,000)	$ (21,000,000)