Note 2 - Significant Accounting Policies and Methods of Application (Detail) (USD $)	11 Months Ended	12 Months Ended
	Dec. 09, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Inventory, LIFO Reserve		$ 190,000,000
Customer Deposits, Current		143,000,000	152,000,000
Goodwill		1,837,000,000	1,813,000,000	418,000,000
Intangible Assets, Net (Excluding Goodwill)		96,000,000	105,000,000
Amortization of Intangible Assets		9,000,000	0
CashCash Equivalents And Short Term Investments Of Foreign Subsidiaries		80,000,000	71,000,000
Excise Taxes Collected		85,000,000	9,000,000
Stock Issued During Period, Shares, Acquisitions (in Shares)	38,200,000		38,200,000
Provision for Doubtful Accounts		63,000,000
Nicor Bad Debt Expense		23,000,000	31,000,000
CustomerRefundLiability		40,000,000	32,000,000
Customer Refundable Fees, Refund Payments		19,000,000
Customer Refund Liability, Current		13,000,000
Asset Retirement Obligation, Period Increase (Decrease)		74,000,000
Regulatory Assets, Current		145,000,000	131,000,000
Regulatory Assets, Noncurrent		944,000,000	1,079,000,000
Recovered Pipeline Replacement Revenue		51,000,000	48,000,000	45,000,000
Regulatory Liability, Current		161,000,000	112,000,000
Nicor Gas [Member]
Inventory, LIFO Reserve		119,000,000
Wholesale Services [Member]
Customer Deposits, Current		10,000,000
Goodwill		0	2,000,000	0
Storage and Fuels Reporting Unit [Member]
Goodwill		14,000,000
Midstream Operations [Member]
Percent Goodwill Exceeds Carrying Value		10.00%
Goodwill		14,000,000	16,000,000	14,000,000
Elizabethtown Gas Infrastructure Program [Member]
Purchase Obligation		40,000,000
Elizabethtown Gas Infrastructure Program Additional Spend [Member]
Purchase Obligation		135,000,000
Minimum [Member]
Percent Goodwill Exceeds Carrying Value		20.00%
Approved Infrastructure Replacement Program		25,000,000
Maximum [Member]
Percent Goodwill Exceeds Carrying Value		500.00%
Approved Infrastructure Replacement Program		105,000,000
Nicor Manufactured Gas Plant Sites in Illinois [Member]
Finite-Lived Intangible Asset, Useful Life		18 years
Atlanta Gas Light [Member]
Regulatory Assets, Current		47,000,000
Regulatory Assets, Noncurrent		167,000,000
Regulatory Liability, Current		$ 121,000,000