Note 6 - Employee Benefit Plans (Detail) - Pension and Other Retirement Plans. (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Defined Benefit Plan Change In Fair Value Of Plan Assets [Member] | Pension Plans [Member]
Change in plan assets
Fair value of plan assets, January 1,	$ 754		$ 344
Plan assets acquired in Nicor merger	0		388
Actual return on plan assets	101		(7)
Employee contributions	0		0
Employer contributions	42		58
Benefits paid	(59)		(28)
Medicare Part D reimbursements	0		0
Plan curtailment and settlements	(1)		(1)
Fair value of plan assets, December 31,	837		754
Defined Benefit Plan Change In Fair Value Of Plan Assets [Member] | Other Retirement Plans [Member]
Change in plan assets
Fair value of plan assets, January 1,	67		71
Plan assets acquired in Nicor merger	0		0
Actual return on plan assets	10		(3)
Employee contributions	1		0
Employer contributions	17		8
Benefits paid	(19)		(9)
Medicare Part D reimbursements	1		0
Plan curtailment and settlements	0		0
Fair value of plan assets, December 31,	77		67
Long Term Asset [Member] | Pension Plans [Member]
Amounts recognized in the Consolidated Statements of Financial Position consist of
Long-term asset	33		26
Long Term Asset [Member] | Other Retirement Plans [Member]
Amounts recognized in the Consolidated Statements of Financial Position consist of
Long-term asset	0		0
Current Liability [Member] | Pension Plans [Member]
Amounts recognized in the Consolidated Statements of Financial Position consist of
Current liability	(2)		(2)
Current Liability [Member] | Other Retirement Plans [Member]
Amounts recognized in the Consolidated Statements of Financial Position consist of
Current liability	(12)		(14)
Long Term Liability [Member] | Pension Plans [Member]
Amounts recognized in the Consolidated Statements of Financial Position consist of
Long-term liability	(240)		(238)
Long Term Liability [Member] | Other Retirement Plans [Member]
Amounts recognized in the Consolidated Statements of Financial Position consist of
Long-term liability	(265)		(316)
Pension Plans [Member]
Change in plan assets
Employee contributions	0		0
Benefits paid	(59)		(28)
Benefit obligation, January 1,	968		531
Benefit obligations acquired in Nicor merger	0		345
Service cost	28		14
Interest cost	44		29
Actuarial loss (gain)	66		78
Plan amendments	0		0
Medicare Part D reimbursements	0		0
Plan curtailment and settlements	(1)		(1)
Benefit obligation, December 31,	1,046		968
Funded status at end of year	(209)		(214)
Amounts recognized in the Consolidated Statements of Financial Position consist of
Total liability at December 31,	(209)		(214)
Accumulated benefit obligation (2)	983	[1]	910	[1]
Supplemental information for underfunded pension plans included above as of December 31, 2012:
Aggregate benefit obligation			604
Aggregate accumulated benefit obligation			570
Aggregate fair value of plan assets			363
Assumptions used to determine benefit obligations
Discount rate	4.20%		4.60%
Rate of compensation increase	3.70%		3.70%
Pension band increase (1)	2.00%	[2]	2.00%	[2]
Other Retirement Plans [Member]
Change in plan assets
Employee contributions	1		0
Benefits paid	(19)		(9)
Benefit obligation, January 1,	397		107
Benefit obligations acquired in Nicor merger	0		273
Service cost	4		1
Interest cost	17		6
Actuarial loss (gain)	(22)		18
Plan amendments	(25)		0
Medicare Part D reimbursements	1		1
Plan curtailment and settlements	0		0
Benefit obligation, December 31,	354		397
Funded status at end of year	(277)		(330)
Amounts recognized in the Consolidated Statements of Financial Position consist of
Total liability at December 31,	(277)		(330)
Accumulated benefit obligation (2)		[1]		[1]
Assumptions used to determine benefit obligations
Discount rate	4.00%		4.50%
Rate of compensation increase	3.70%		3.70%
Pension band increase (1)		[2]		[2]

[1] ABO differs from the projected benefit obligation in that the ABO excludes the effect of salary and wage increases.

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