Rate And Regulatory Matters (Schedule Of Regulatory Assets And Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Rate And Regulatory Matters [Line Items]
Current regulatory assets	$ 156		$ 247
Noncurrent regulatory assets	1,240		1,786
Current regulatory liabilities	216		100
Noncurrent regulatory liabilities	1,705		1,589
Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	118		163
Noncurrent regulatory assets	534		852
Current regulatory liabilities	57		18
Noncurrent regulatory liabilities	1,041		917
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	38		84
Noncurrent regulatory assets	701		934
Current regulatory liabilities	159		82
Noncurrent regulatory liabilities	664		672
Under-Recovered FAC
Rate And Regulatory Matters [Line Items]
Current regulatory assets	104	[1],[2]	145	[1],[2]
Under-Recovered FAC | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	104	[1],[2]	145	[1],[2]
Under-Recovered FAC | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	0	[1],[2]	0	[1],[2]
Under-Recovered Illinois Electric Power Costs
Rate And Regulatory Matters [Line Items]
Current regulatory assets	1	[3]	0	[3]
Under-Recovered Illinois Electric Power Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets		[3]		[3]
Under-Recovered Illinois Electric Power Costs | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	1	[3]	0	[3]
Under-Recovered PGA
Rate And Regulatory Matters [Line Items]
Current regulatory assets	1	[3]	12	[3]
Under-Recovered PGA | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	0	[3]	5	[3]
Under-Recovered PGA | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	1	[3]	7	[3]
MTM Derivative Losses
Rate And Regulatory Matters [Line Items]
Current regulatory assets	50	[4]	90	[4]
Noncurrent regulatory assets	126	[4]	135	[4]
MTM Derivative Losses | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	14	[4]	13	[4]
Noncurrent regulatory assets	8	[4]	7	[4]
MTM Derivative Losses | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	36	[4]	77	[4]
Noncurrent regulatory assets	118	[4]	128	[4]
Pension And Postretirement Benefit Costs
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	184	[5]	772	[5]
Pension And Postretirement Benefit Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	44	[5]	348	[5]
Pension And Postretirement Benefit Costs | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	140	[5]	424	[5]
Income Taxes
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	237	[6]	235	[6]
Noncurrent regulatory liabilities	41	[7]	46	[7]
Income Taxes | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	230	[6]	231	[6]
Noncurrent regulatory liabilities	38	[7]	42	[7]
Income Taxes | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	7	[6]	4	[6]
Noncurrent regulatory liabilities	3	[7]	4	[7]
Asset Retirement Obligation
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	5	[8]	5	[8]
Noncurrent regulatory liabilities	146	[8]	80	[8]
Asset Retirement Obligation | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets		[8]	0	[8]
Noncurrent regulatory liabilities	146	[8]	80	[8]
Asset Retirement Obligation | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	5	[8]	5	[8]
Noncurrent regulatory liabilities		[8]	0	[8]
Callaway Costs
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	40	[1],[9]	44	[1],[9]
Callaway Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	40	[1],[9]	44	[1],[9]
Callaway Costs | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets		[1],[9]		[1],[9]
Unamortized Loss On Reacquired Debt
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	151	[1],[10]	181	[1],[10]
Unamortized Loss On Reacquired Debt | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	77	[1],[10]	81	[1],[10]
Unamortized Loss On Reacquired Debt | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	74	[1],[10]	100	[1],[10]
Recoverable Costs Contaminated Facilities
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	271	[11]	248	[11]
Recoverable Costs Contaminated Facilities | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets		[11]	0	[11]
Recoverable Costs Contaminated Facilities | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	271	[11]	248	[11]
Storm Costs
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	8	[12]	9	[12]
Storm Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	5	[12]	9	[12]
Storm Costs | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	3	[12]	0	[12]
Demand-Side Costs
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	58	[1],[13]	73	[1],[13]
Demand-Side Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	58	[1],[13]	73	[1],[13]
Demand-Side Costs | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets		[1],[13]	0	[1],[13]
Reserve For Workers' Compensation Liabilities
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	12	[14]	12	[14]
Reserve For Workers' Compensation Liabilities | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	6	[14]	6	[14]
Reserve For Workers' Compensation Liabilities | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	6	[14]	6	[14]
Bad Debt Rider
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory liabilities	8	[15]	12	[15]
Bad Debt Rider | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory liabilities		[15]		[15]
Bad Debt Rider | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory liabilities	8	[15]	12	[15]
Credit Facilities Fees
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	5	[16]	6	[16]
Credit Facilities Fees | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	5	[16]	6	[16]
Credit Facilities Fees | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets		[16]	0	[16]
Common Stock Issuance Costs
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	4	[17]	7	[17]
Common Stock Issuance Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	4	[17]	7	[17]
Common Stock Issuance Costs | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets		[17]	0	[17]
Construction Accounting For Pollution Control Equipment
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	22	[1],[18]	23	[1],[18]
Construction Accounting For Pollution Control Equipment | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	22	[1],[18]	23	[1],[18]
Construction Accounting For Pollution Control Equipment | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets		[1],[18]	0	[1],[18]
Solar Rebates
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	27	[1],[19]	5	[1],[19]
Solar Rebates | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	27	[1],[19]	5	[1],[19]
Solar Rebates | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	0	[1],[20]	0	[1],[19]
IEMA Revenue Requirement Reconciliation
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	65	[21]	0	[21]
Current regulatory liabilities	65	[21]	0	[21]
Noncurrent regulatory liabilities	0	[21]	55	[21]
IEMA Revenue Requirement Reconciliation | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	0	[21]	0	[21]
Current regulatory liabilities	0	[21]	0	[21]
Noncurrent regulatory liabilities	0	[21]	0	[21]
IEMA Revenue Requirement Reconciliation | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	65	[21]	0	[21]
Current regulatory liabilities	65	[21]	0	[21]
Noncurrent regulatory liabilities	0	[21]	55	[21]
Other
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	25	[20],[22]	31	[20]
Noncurrent regulatory liabilities	10	[23]	4	[23]
Other | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	8	[20]	12	[20]
Noncurrent regulatory liabilities	10	[23]	4	[23]
Other | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	12	[20]	19	[20]
Noncurrent regulatory liabilities	0	[23]	0	[23]
Over-Recovered FAC
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	26	[2]	0	[2]
Over-Recovered FAC | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	26	[2]	0	[2]
Over-Recovered FAC | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	0	[2]	0	[2]
Over-Recovered Illinois Electric Power Costs
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	51	[3]	58	[3]
Over-Recovered Illinois Electric Power Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	0	[3]	0	[3]
Over-Recovered Illinois Electric Power Costs | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	51	[3]	58	[3]
Over-Recovered PGA
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	34	[3]	15	[3]
Over-Recovered PGA | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	5	[3]	0	[3]
Over-Recovered PGA | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	29	[3]	15	[3]
MTM Derivative Gains
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	27	[4]	19	[4]
Noncurrent regulatory liabilities	1	[4]	2	[4]
MTM Derivative Gains | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	26	[4]	18	[4]
Noncurrent regulatory liabilities	1	[4]	2	[4]
MTM Derivative Gains | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	1	[4]	1	[4]
Noncurrent regulatory liabilities		[4]		[4]
Wholesale Distribution Refund
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	13	[24]	8	[24]
Wholesale Distribution Refund | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	0	[24]	0	[24]
Wholesale Distribution Refund | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	13	[24]	8	[24]
Removal Costs
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory liabilities	1,438	[25]	1,347	[25]
Removal Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory liabilities	828	[25]	766	[25]
Removal Costs | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory liabilities	610	[25]	581	[25]
Pension And Postretirement Benefit Costs Tracker
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory liabilities	15	[26]	23	[26]
Pension And Postretirement Benefit Costs Tracker | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory liabilities	15	[26]	23	[26]
Pension And Postretirement Benefit Costs Tracker | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory liabilities	0	[26]	0	[26]
Energy Efficiency Rider
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory liabilities	36	[27]	20	[27]
Energy Efficiency Rider | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory liabilities	3	[27]	0	[27]
Energy Efficiency Rider | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory liabilities	33	[27]	20	[27]
FERC Revenue Requirement Reconciliation
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory liabilities	10	[28]	0	[28]
FERC Revenue Requirement Reconciliation | Union Electric Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory liabilities	0	[28]	0	[28]
FERC Revenue Requirement Reconciliation | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory liabilities	$ 10	[28]	$ 0	[28]

[1] These assets earn a return.

[2] Under-recovered or over-recovered fuel costs to be recovered through the FAC. Specific accumulation periods aggregate the under-recovered or over-recovered costs over four months, any related adjustments that occur over the following four months, and the recovery from customers that occurs over the next eight months.

[3] Costs under- or over-recovered from utility customers. Amounts will be recovered from, or refunded to, customers within one year of the deferral.

[4] Deferral of commodity-related derivative MTM losses or gains. See Note 7 – Derivative Financial Instruments for additional information.

[5] These costs are being amortized in proportion to the recognition of prior service costs (credits), transition obligations (assets), and actuarial losses (gains) attributable to Ameren’s pension plan and postretirement benefit plans. See Note 11 – Retirement Benefits for additional information.

[6] Offset to certain deferred tax liabilities for expected recovery of future income taxes when paid. This will be recovered over the expected life of the related assets.

[7] Unamortized portion of investment tax credits, federal excess deferred taxes, and uncertain tax position tracker. The tracker is being amortized over three years, beginning in January 2013. The unamortized portion of investment tax credit is being amortized over the expected life of the underlying assets.

[8] Recoverable or refundable removal costs for AROs, including net realized and unrealized gains and losses related to the nuclear decommissioning trust fund investments. See Note 1 – Summary of Significant Accounting Policies – Asset Retirement Obligations.

[9] Ameren Missouri’s Callaway energy center operations and maintenance expenses, property taxes, and carrying costs incurred between the plant in-service date and the date the plant was reflected in rates. These costs are being amortized over the remaining life of the energy center's current operating license, which expires in 2024.

[10] Losses related to reacquired debt. These amounts are being amortized over the lives of the related new debt issuances or the original lives of the old debt issuances if no new debt was issued.

[11] The recoverable portion of accrued environmental site liabilities that will be collected from electric and natural gas customers through ICC-approved cost recovery riders. The period of recovery will depend on the timing of remediation expenditures. See Note 15 – Commitments and Contingencies for additional information.

[12] Actual storm costs in a test year that exceed the MoPSC staff’s normalized storm costs for rate purposes. As approved by the December 2012 MoPSC electric rate order, the 2006, 2007, and 2008 storm costs are being amortized through December 2014. As approved by the May 2010 MoPSC electric rate order, the 2009 storm costs are being amortized through June 2015. The Ameren Illinois total includes 2013 storm costs deferred in accordance with the IEIMA. These costs are being amortized over a five-year period beginning in 2013.

[13] Demand-side costs incurred prior to implementation of the MEEIA in 2013, including the costs of developing, implementing and evaluating customer energy efficiency and demand response programs. Costs incurred from May 2008 through September 2008 are being amortized over a 10-year period that began in March 2009. Costs incurred from October 2008 through December 2009 are being amortized over a six-year period that began in July 2010. Costs incurred from January 2010 through February 2011 are being amortized over a six-year period that began in August 2011. Costs incurred from March 2011 through July 2012 are being amortized over a six-year period that began in January 2013.

[14] Reserve for workers’ compensation claims. The period of recovery will depend on the timing of actual expenditures.

[15] A regulatory tracking mechanism for the difference between the level of bad debt expense incurred by Ameren Illinois under GAAP and the level of such costs included in electric and natural gas rates. The over-recovery relating to 2011 was refunded to customers from June 2012 through May 2013. The over-recovery relating to 2012 is being refunded to customers from June 2013 through May 2014. The over-recovery relating to 2013 will be refunded to customers from June 2013 through May 2014.

[16] Ameren Missouri’s costs incurred to enter into and maintain the 2012 Ameren Missouri Credit Agreement. These costs are being amortized over five years, beginning in November 2012. These costs are being amortized to construction work in progress, which will be depreciated when assets are placed into service.

[17] The MoPSC’s May 2010 electric rate order allowed Ameren Missouri to recover its portion of Ameren’s September 2009 common stock issuance costs. These costs are being amortized over five years, beginning in July 2010.

[18] The MoPSC’s May 2010 electric rate order allowed Ameren Missouri to record an allowance for funds used during construction for pollution control equipment at its Sioux energy center until the cost of that equipment could be included in customer rates. These costs will be amortized over the expected life of the Sioux energy center, which is currently through 2033.

[19] Costs associated with Ameren Missouri's solar rebate program beginning in August 2012 to fulfill Ameren Missouri's renewable energy portfolio requirement. The amortization period for these costs will be three years, commencing with the next Ameren Missouri electric rate case order.

[20] The Ameren Illinois total includes Ameren Illinois Merger integration and optimization costs, which are amortized over four years, beginning in January 2012. The Ameren Illinois total also includes costs related to the 2013 natural gas delivery service rate case costs, which are being amortized over a two-year period that began in January 2014. The Ameren Illinois total also includes a portion of the unamortized debt fair value adjustment recorded upon Ameren's acquisition of IP. This portion is being amortized over the remaining life of the related debt. At Ameren Missouri, the balance primarily includes the cost of renewable energy credits to fulfill its renewable energy portfolio requirement. Costs incurred from January 2010 through July 2012 are being amortized over three years, beginning in January 2013.

[21] The asset balance relates to the difference between Ameren Illinois' 2013 revenue requirement calculated under the IEIMA's performance-based formula ratemaking framework, and the revenue requirement included in customer rates for 2013. Subject to ICC approval, this asset will be collected from customers in 2015. The liability balance relates to the difference between Ameren Illinois' 2012 revenue requirement calculated under the IEIMA's performance-based formula ratemaking framework and the revenue requirement included in customer rates for 2012. This liability will be refunded to customers in 2014.

[22] The Ameren total includes $5 million for ATXI's revenue requirement reconciliation adjustments for 2012 and 2013 calculated pursuant to the FERC's electric transmission formula ratemaking framework. These adjustments will be collected from customers in 2014 for the 2012 revenue requirement reconciliation and in 2015 for the 2013 revenue requirement reconciliation.

[23] Balance primarily includes the costs of renewable energy credits to fulfill Ameren Missouri's renewable energy portfolio requirement from August 2012 through December 2013, which were less than the amount included in rates. The amortization period for this over-recovery will be determined in a future Ameren Missouri electric rate case.

[24] Estimated refund to wholesale electric customers. See 2011 Wholesale Distribution Rate Case above.

[25] Estimated funds collected for the eventual dismantling and removal of plant from service, net of salvage value, upon retirement related to our rate-regulated operations.

[26] A regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs built into rates. For periods prior to August 2012, the MoPSC's December 2012 electric rate order directed the amortization to occur over five years, beginning in January 2013. For periods after August 2012, the amortization period will be determined in a future Ameren Missouri electric rate case.

[27] The Ameren Illinois balance relates its regulatory tracking mechanism to recover its electric and natural gas costs associated with developing, implementing, and evaluating customer energy efficiency and demand response programs. This over-recovery will be refunded to customers over the following 12 months after the plan year. The Ameren Missouri balance relates to its MEEIA program costs incurred and projected lost revenues compared to the amount previously collected from customers. Beginning in January 2014, a MEEIA rider allows Ameren Missouri to collect from or refund to customers any annual difference in the actual amounts incurred and the projected amounts collected from customers for the MEEIA program costs and its projected lost revenues. Under the MEEIA rider, collections from or refunds to customers occur one year after the program costs and projected lost revenues are incurred.

[28] Ameren Illinois' 2013 revenue requirement reconciliation adjustment calculated pursuant to the FERC's electric transmission formula ratemaking framework. This liability will be refunded to customers in 2015.