Note 2 - Significant Accounting Policies and Methods of Application (Detail) - Summary of Regulatory Liabilities (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011		Sep. 30, 2011
Regulatory liabilities
Regulatory liabilities - current	$ 117	[1]	$ 112	[2]	$ 34
Regulatory liabilities
Regulatory liabilities - noncurrent	1,465	[1]	1,405	[2]	296
Total regulatory liabilities	1,582	[1]	1,517	[2]	330
Accrued natural gas costs [Member]
Regulatory liabilities
Regulatory liabilities - current	62	[1]	53	[2]	26
Bad debt rider [Member]
Regulatory liabilities
Regulatory liabilities - current	30	[1]	30	[2]	0
Regulatory liabilities
Regulatory liabilities - noncurrent	16	[1]	14	[2]	0
Accumulated removal costs [Member]
Regulatory liabilities
Regulatory liabilities - current	14	[1]	14	[2]	0
Regulatory liabilities
Regulatory liabilities - noncurrent	1,381	[1]	1,321	[2]	254
Other Regulatory Liability [Member]
Regulatory liabilities
Regulatory liabilities - current	11	[1]	15	[2]	8
Regulatory liabilities
Regulatory liabilities - noncurrent	15	[1]	11	[2]	17
Unamortized investment tax credit [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	30	[1]	32	[2]	11
Regulatory income tax liability [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	$ 23	[1]	$ 27	[2]	$ 14

[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.