Note 2 - Significant Accounting Policies and Methods of Application (Detail) - Summary of Regulatory Assets (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011		Mar. 31, 2011
Regulatory assets - current
Regulatory assets - current	$ 137	$ 131	[1]	$ 73
Regulatory assets - long-term
Regulatory assets - long-term	1,057	1,079	[1]	434
Total regulatory assets	1,194	1,210	[1]	507
Recoverable regulatory infrastructure program costs [Member]
Regulatory assets - current
Regulatory assets - current	48	48	[1]	49
Regulatory assets - long-term
Regulatory assets - long-term	291	305	[1]	231
Recoverable Retirement Benefit Costs [Member]
Regulatory assets - current
Regulatory assets - current	29	29	[1]	0
Regulatory assets - long-term
Regulatory assets - long-term	256	262	[1]	9
Recoverable ERC [Member]
Regulatory assets - current
Regulatory assets - current	7	7	[1]	7
Regulatory assets - long-term
Regulatory assets - long-term	349	351	[1]	162
Other Regulatory Assets [Member]
Regulatory assets - current
Regulatory assets - current	53	47	[1]	17
Regulatory assets - long-term
Regulatory assets - long-term	140	140	[1]	22
Unamortized losses on reacquired debt [Member]
Regulatory assets - long-term
Regulatory assets - long-term	$ 21	$ 21	[1]	$ 10

[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.