Note 2 - Significant Accounting Policies and Methods of Application (Detail) - Summary of Regulatory Liabilities (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011		Mar. 31, 2011
Regulatory liabilities - current
Regulatory liabilities - current	$ 173	$ 112	[1]	$ 77
Regulatory liabilities - long-term
Regulatory liabilities - long-term	1,431	1,405	[1]	296
Total regulatory liabilities	1,604	1,517	[1]	373
Accrued natural gas costs [Member]
Regulatory liabilities - current
Regulatory liabilities - current	97	53	[1]	51
Bad debt rider [Member]
Regulatory liabilities - current
Regulatory liabilities - current	32	30	[1]	0
Regulatory liabilities - long-term
Regulatory liabilities - long-term	20	14	[1]	0
Accumulated removal costs [Member]
Regulatory liabilities - current
Regulatory liabilities - current	14	14	[1]	0
Regulatory liabilities - long-term
Regulatory liabilities - long-term	1,339	1,321	[1]	250
Other Regulatory Liability [Member]
Regulatory liabilities - current
Regulatory liabilities - current	30	15	[1]	26
Regulatory liabilities - long-term
Regulatory liabilities - long-term	14	11	[1]	20
Unamortized investment tax credit [Member]
Regulatory liabilities - long-term
Regulatory liabilities - long-term	32	32	[1]	11
Regulatory income tax liability [Member]
Regulatory liabilities - long-term
Regulatory liabilities - long-term	$ 26	$ 27	[1]	$ 15

[1] The increase in regulatory assets and liabilities from March 31, 2011, includes $545 million related to the addition of Nicor Gas' regulatory assets and includes $1,330 million related to the addition of Nicor Gas' regulatory liabilities.