Note 6 - Employee Benefit Plans (Detail) - Employee Benefit Plans (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Pension Plans, Defined Benefit [Member]
Service cost	$ 28,000,000		$ 14,000,000		$ 11,000,000
Interest cost	44,000,000		29,000,000		27,000,000
Expected return on plan assets	(64,000,000)		(33,000,000)		(28,000,000)
Net amortization of prior service credit	(2,000,000)		(2,000,000)		(2,000,000)
Recognized actuarial loss	34,000,000		14,000,000		10,000,000
Net periodic benefit cost	40,000,000		22,000,000		18,000,000
Discount rate (1)	4.60%	[1]	5.40%	[1]	6.00%	[1]
Expected return on plan assets (1)	8.40%	[1]	8.50%	[1]	8.80%	[1]
Rate of compensation increase(1)	3.70%	[1]	3.70%	[1]	3.70%	[1]
Pension band increase (1) (2)	0.020	[1],[2]	0.020	[1],[2]		[1],[2]
Other Postretirement Benefit Plans, Defined Benefit [Member]
Service cost	4,000,000		1,000,000		0
Interest cost	16,000,000		6,000,000		6,000,000
Expected return on plan assets	(5,000,000)		(5,000,000)		(5,000,000)
Net amortization of prior service credit	(3,000,000)		(4,000,000)		(4,000,000)
Recognized actuarial loss	9,000,000		2,000,000		2,000,000
Net periodic benefit cost	21,000,000		0		(1,000,000)
Discount rate (1)	4.50%	[1]	5.20%	[1]	5.80%	[1]
Expected return on plan assets (1)	8.50%	[1]	8.20%	[1]	8.80%	[1]
Rate of compensation increase(1)	3.80%	[1]	3.70%	[1]	3.70%	[1]
Pension band increase (1) (2)		[1],[2]		[1],[2]		[1],[2]

[1] Rates are presented on a weighted average basis.

[2] Only applicable to the Nicor Gas pension plan.