Note 3 - Merger with Nicor (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended	10 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 Nicor Gas [Member]
Business Acquisition, Effective Date of Acquisition					Dec. 09, 2011
Business Combination, Consideration Transferred					$ 2,500,000,000
Liabilities, Fair Value Adjustment						99,000,000
Assets, Fair Value Adjustment						28,000,000
Business Combination, Separately Recognized Transactions, Expenses and Losses Recognized	2,000,000	2,000,000	15,000,000	12,000,000
Business Combination Separately Recognized Transactions Expenses And Losses Recognized Net Tax	1,000,000	1,000,000	9,000,000	7,000,000
Merger Related Expenses, Debt Issuance and Interest Expense		6,000,000		14,000,000
Merger Related Expenses, Debt Issuance and Interest Expense, Net		4,000,000		9,000,000
Merger Related Expenses				$ 29,000,000	$ 96,000,000