Note 2 - Significant Accounting Policies and Methods of Application (Detail) - Summary of Regulatory Assets (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011		Sep. 30, 2011
Regulatory assets
Regulatory assets - current	$ 158	[1]	$ 131	[2]	$ 99
Regulatory assets
Regulatory assets - noncurrent	1,086	[1]	1,079	[2]	532
Total regulatory assets	1,244	[1]	1,210	[2]	631
Recoverable regulatory infrastructure program costs [Member]
Regulatory assets
Regulatory assets - current	46	[1]	48	[2]	62
Regulatory assets
Regulatory assets - noncurrent	286	[1]	305	[2]	269
Recoverable ERC [Member]
Regulatory assets
Regulatory assets - current	33	[1]	7	[2]	7
Regulatory assets
Regulatory assets - noncurrent	421	[1]	351	[2]	213
Recoverable Retirement Benefit Costs [Member]
Regulatory assets
Regulatory assets - current	27	[1]	29	[2]	0
Regulatory assets
Regulatory assets - noncurrent	228	[1]	263	[2]	10
Other Regulatory Assets [Member]
Regulatory assets
Regulatory assets - current	52	[1]	47	[2]	30
Regulatory assets
Regulatory assets - noncurrent	59	[1]	61	[2]	40
Long-Term Debt Fair Value Adjustment [Member]
Regulatory assets
Regulatory assets - noncurrent	$ 92	[1]	$ 99	[2]	$ 0

[1] The increase in regulatory assets and liabilities from December 31, 2011, includes an increase in recoverable ERC due to revised estimates of our ERC liabilities made during the second quarter of 2012 of $103 million, which primarily related to Nicor Gas' former manufactured gas plant sites in Illinois. See Note 9 - Commitments, Guarantees and Contingencies for additional ERC disclosures.

[2] The increase in regulatory assets and liabilities from September 30, 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.