Employee Benefits Plan (Tables)

12 Months Ended

Dec. 31, 2011

Notes To Financial Statements [Abstract]

Schedule of Defined Benefit Plans Disclosures [Table]

The following tables present details about our pension and other retirement plans. 

	Pension plans		Other retirement plans
Dollars in millions	2011	2010	2011	2010
Change in plan assets
Fair value of plan assets, January 1,	$344	$303	$71	$63
Plan assets acquired in Nicor merger	388	0	0	0
Actual return on plan assets	(7)	37	(3)	8
Employer contributions	58	31	8	7
Benefits paid	(28)	(27)	(9)	(7)
Plan curtailment and settlements	(1)	0	0	0
Fair value of plan assets, December 31,	$754	$344	$67	$71
Change in benefit obligation
Benefit obligation, January 1,	$531	$463	$107	$101
Benefit obligations acquired in Nicor merger	345	0	273	0
Service cost	14	11	1	0
Interest cost	29	27	6	6
Actuarial loss	78	57	18	7
Medicare Part D reimbursements	0	0	1	0
Benefits paid	(28)	(27)	(9)	(7)
Plan curtailment and settlements	(1)	0	0	0
Benefit obligation, December 31,	$968	$531	$397	$107
Funded status at end of year	$(214)	$(187)	$(330)	$(36)
Amounts recognized in the Consolidated Statements of Financial Position consist of
Long-term asset	$26	$0	$0	$0
Current liability	(2)	(1)	(14)	0
Long-term liability	(238)	(186)	(316)	(36)
Total liability at December 31,	$(214)	$(187)	$(330)	$(36)
Accumulated benefit obligation (2)	$910	$506	n/a	n/a
Supplemental information for underfunded pension plans included above as of December 31, 2011:
Aggregate benefit obligation	$604	$531	n/a	n/a
Aggregate accumulated benefit obligation	570	506	n/a	n/a
Aggregate fair value of plan assets	$363	$344	n/a	n/a
Assumptions used to determine benefit obligations
Discount rate	4.6%	5.4%	4.5%	5.2%
Rate of compensation increase	3.7%	3.7%	3.7%	3.7%
Pension band increase (1)	2.0%	n/a	n/a	n/a

(1)Only applicable to the Nicor Gas pension plan

(2) ABO differs from the projected benefit obligation in that the ABO excludes the effect of salary and wage increases.

Schedule Of Health Care Cost Trend Rates [Table Text Block]

Assumptions used to determine the 2012 health care benefit cost for the Nicor Gas Welfare Benefit Plan were as follows:

	2011
Health care cost trend rate assumed for next year	8.60%
Ultimate rate to which the cost trend rate is assumed to decline	4.50%
Year that reaches ultimate trend rate	2030

Schedule Of Effect Of One Percentage Point Change In Assumed Health Care cost Trend Rates [Table Text Block]

Assumed health care cost trend rates can have a significant effect on the amounts reported for the health care plans. A one-percentage-point change in the assumed health care cost trend rates for the Nicor Gas Welfare Benefit Plan would have the following effects:

Dollars in millions	Effect on service and interest cost	Effect on benefit obligation
1% Health care cost trend rate increase	$0	$26
1% Health care cost trend rate decrease	0	(22)

Schedule Of Net Benefit Costs [Table Text Block]

The components of our pension and other retirement benefit costs are set forth in the following table.

	Pension plans			Other retirement plans
Dollars in millions	2011	2010	2009	2011	2010	2009
Net benefit cost
Service cost	$14	$11	$8	$1	$0	$0
Interest cost	29	27	26	6	6	6
Expected return on plan assets	(33)	(28)	(29)	(5)	(5)	(4)
Net amortization of prior service cost	(2)	(2)	(2)	(4)	(4)	(4)
Recognized actuarial loss	14	10	9	2	2	2
Net periodic benefit cost	$22	$18	$12	$0	$(1)	$0
Assumptions used to determine benefit costs
Discount rate (1)	5.4%	6.0%	6.2%	5.2%	5.8%	6.2%
Expected return on plan assets (1)	8.5%	8.8%	9.0%	8.0%	8.8%	9.0%
Rate of compensation increase(1)	3.7%	3.7%	3.7%	3.7%	3.7%	3.7%
Pension band increase (1) (2)	2.0%	n/a	n/a	n/a	n/a	n/a

(1)Rates are presented on a weighted average basis

(2)Only applicable to the Nicor Gas pension plan.

Schedule Of Amounts In Accumulated Other Comprehensive Income Loss To Be Recognized Over Next Fiscal Year [Table Text Block]

The 2012 estimated amortization out of regulatory assets or accumulated OCI for these plans are set forth in the following table.

	Net Regulatory Asset		Accumulated OCI		Total
In millions	Pension plans	Other retirement plans	Pension plans	Other retirement plans	Pension plans	Other retirement plans
Amortization of prior service credit	$0	$0	$(2)	$(3)	$(2)	$(3)
Amortization of net loss	14	8	20	3	33	11

Schedule Of Expected Benefit Payments [Table Text Block]

The following table presents the gross benefit payments expected for the years ended December 31, 2012 through 2021 for our pension and other retirement plans. There will be benefit payments under these plans beyond 2021.

In millions	Pension plans	Other retirement plans	Expected Medicare subsidy
2012	$56	$23	$2
2013	55	23	2
2014	59	24	2
2015	62	25	2
2016	64	26	2
2017-2021	367	144	12

Schedule Of Net Periodic Benefit Cost Not Yet Recognized [Table Text Block]

The following table presents the amounts not yet reflected in net periodic benefit cost and included in net regulatory assets and accumulated OCI as of December 31, 2011 and 2010:

	Net regulatory assets		Accumulated OCI		Total
In millions	Pension plan	Other retirement plans	Pension plan	Other retirement plans	Pension plan	Other retirement plans
December 31, 2011:
Prior service cost (credit)	$1	$1	$(13)	$(4)	$(12)	$(3)
Net loss	162	118	312	51	474	170
Total	$163	$119	$299	$47	$462	$167
December 31, 2010:
Prior service credit	$0	$0	$(15)	$(8)	$(15)	$(8)
Net loss	0	0	226	35	226	35
Total	$0	$0	$211	$27	$211	$27