Benefits - Narrative (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 25, 2012	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Allocation Percentage, Cash Minimum			0.00%
Defined Benefit Plan, Target Allocation Percentage, Cash Maximum			10.00%
Other Comprehensive Income (Loss), Amortization, Pension and Other Postretirement Benefit Plans, Net Prior Service Cost (Credit) Recognized in Net Periodic Benefit Cost, before Tax			1
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year		8	8
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Prior Service Cost Arising During Period, before Tax			1
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations Range Minimum			10.00%
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum			35.00%
Defined Benefit Plan, Target Allocation Percentage, Cash Minimum			0.00%
Defined Benefit Plan, Target Allocation Percentage, Cash Maximum			10.00%
Other Comprehensive Income (Loss), Reclassification, Pension and Other Postretirement Benefit Plans, Net Gain (Loss) Recognized in Net Periodic Benefit Cost, before Tax			2
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year		18	18
Savings Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Matching Contribution Minimum					2.00%
Matching Contribution Maximum			6.25%
Company Contributions Vested			100.00%
Company Contributions Years			five
Supplemental Employee Retirement Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Matching Contribution Minimum					3.50%
Matching Contribution Maximum			12.00%
Company Contributions Vested			100.00%
Equity Securities [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations Range Minimum			25.00%
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum			35.00%
Equity Securities [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations Range Minimum			25.00%
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum			70.00%
Fixed Income Securities [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations Range Minimum			65.00%
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum			75.00%
Fixed Income Securities [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations Range Minimum			15.00%
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum			35.00%
Savings Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Cost Recognized	2	6		8	7
Savings Plan [Member] | Missouri Gas Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Company Contributions Vested			100.00%
Company Contributions Years			six
Supplemental Employee Retirement Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Company Contributions Years			five
Defined Contribution Plan, Cost Recognized	$ 3	$ 2		$ 8	$ 8