Note 2 - Significant Accounting Policies and Methods of Application (Detail) - Summary of Regulatory Assets (USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011		Jun. 30, 2011
Regulatory assets - current
Regulatory assets - current	$ 146	[1]	$ 131	[2]	$ 71
Regulatory assets - long-term
Regulatory assets - long-term	1,128	[1]	1,079	[2]	543
Total regulatory assets	1,274	[1]	1,210	[2]	614
Recoverable regulatory infrastructure program costs [Member]
Regulatory assets - current
Regulatory assets - current	47	[1]	48	[2]	47
Regulatory assets - long-term
Regulatory assets - long-term	313	[1]	305	[2]	281
Recoverable Retirement Benefit Costs [Member]
Regulatory assets - current
Regulatory assets - current	27	[1]	29	[2]	0
Regulatory assets - long-term
Regulatory assets - long-term	232	[1]	262	[2]	9
Recoverable ERC [Member]
Regulatory assets - current
Regulatory assets - current	30	[1]	7	[2]	7
Regulatory assets - long-term
Regulatory assets - long-term	428	[1]	351	[2]	215
Other Regulatory Assets [Member]
Regulatory assets - current
Regulatory assets - current	42	[1]	47	[2]	17
Regulatory assets - long-term
Regulatory assets - long-term	41	[1]	41	[2]	28
Long-Term Debt Fair Value Adjustment [Member]
Regulatory assets - long-term
Regulatory assets - long-term	94	[1]	99	[2]	0
Unamortized losses on reacquired debt [Member]
Regulatory assets - long-term
Regulatory assets - long-term	$ 20	[1]	$ 21	[2]	$ 10

[1] The increase in regulatory assets and liabilities from December 31, 2011, includes an increase in our recoverable ERC due to a $103 million increase in the estimates of our ERC liabilities primarily related to Nicor Gas' former operating sites in Illinois. See Note 9 - Commitments, Guarantees and Contingencies for additional ERC disclosures.

[2] The increase in regulatory assets and liabilities from June 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.