Rate And Regulatory Matters (Schedule Of Regulatory Assets And Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Rate And Regulatory Matters [Line Items]
Current regulatory assets	$ 295		$ 156
Regulatory assets	1,582		1,240
Current regulatory liabilities	106		216
Regulatory liabilities	1,850		1,705
Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	163		118
Regulatory assets	695		534
Current regulatory liabilities	18		57
Regulatory liabilities	1,147		1,041
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	129		38
Regulatory assets	883		701
Current regulatory liabilities	84		159
Regulatory liabilities	703		664
Under-Recovered FAC
Rate And Regulatory Matters [Line Items]
Current regulatory assets	128	[1],[2]	104	[1],[2]
Under-Recovered FAC | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	128	[1],[2]	104	[1],[2]
Under-Recovered FAC | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	0	[1],[2]
Under-Recovered Illinois Electric Power Costs
Rate And Regulatory Matters [Line Items]
Current regulatory assets	2	[3]	1	[3]
Under-Recovered Illinois Electric Power Costs | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	2	[3]	1	[3]
Under-Recovered PGA
Rate And Regulatory Matters [Line Items]
Current regulatory assets	20	[3]	1	[3]
Under-Recovered PGA | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	20	[3]	1	[3]
MTM Derivative Losses
Rate And Regulatory Matters [Line Items]
Current regulatory assets	74	[4]	50	[4]
Regulatory assets	158	[4]	126	[4]
MTM Derivative Losses | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	32	[4]	14	[4]
Regulatory assets	14	[4]	8	[4]
MTM Derivative Losses | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	42	[4]	36	[4]
Regulatory assets	144	[4]	118	[4]
Energy Efficiency Rider
Rate And Regulatory Matters [Line Items]
Current regulatory assets	3	[5]
Regulatory liabilities	39	[5]	36	[5]
Energy Efficiency Rider | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	3	[5]
Regulatory liabilities			3	[5]
Energy Efficiency Rider | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	39	[5]	33	[5]
IEMA Revenue Requirement Reconciliation
Rate And Regulatory Matters [Line Items]
Current regulatory assets	65	[1],[6]
Regulatory assets	101	[1],[6]	65	[1],[6]
Current regulatory liabilities			65	[6]
IEMA Revenue Requirement Reconciliation | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	65	[1],[6]
Regulatory assets	101	[1],[6]	65	[1],[6]
Current regulatory liabilities			65	[6]
FERC Revenue Requirement Reconciliation
Rate And Regulatory Matters [Line Items]
Current regulatory assets	3	[1],[7]
Regulatory assets	12	[1],[7]	5	[1],[7]
Current regulatory liabilities	11	[7]
Regulatory liabilities			10	[7]
FERC Revenue Requirement Reconciliation | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	8	[1],[7]
Current regulatory liabilities	11	[7]
Regulatory liabilities			10	[7]
Pension And Postretirement Benefit Costs
Rate And Regulatory Matters [Line Items]
Regulatory assets	423	[8]	184	[8]
Pension And Postretirement Benefit Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	148	[8]	44	[8]
Pension And Postretirement Benefit Costs | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	275	[8]	140	[8]
Income Taxes
Rate And Regulatory Matters [Line Items]
Regulatory assets	256	[9]	237	[9]
Regulatory liabilities	55	[10]	40	[10]
Income Taxes | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	253	[9]	230	[9]
Regulatory liabilities	41	[10]	37	[10]
Income Taxes | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	3	[9]	7	[9]
Regulatory liabilities	14	[10]	3	[10]
Asset Retirement Obligation
Rate And Regulatory Matters [Line Items]
Regulatory assets	5	[11]	5	[11]
Regulatory liabilities	182	[11]	146	[11]
Asset Retirement Obligation | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	182	[11]	146	[11]
Asset Retirement Obligation | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	5	[11]	5	[11]
Callaway Costs
Rate And Regulatory Matters [Line Items]
Regulatory assets	36	[1],[12]	40	[1],[12]
Callaway Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	36	[1],[12]	40	[1],[12]
Unamortized Loss On Reacquired Debt
Rate And Regulatory Matters [Line Items]
Regulatory assets	152	[1],[13]	151	[1],[13]
Unamortized Loss On Reacquired Debt | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	72	[1],[13]	77	[1],[13]
Unamortized Loss On Reacquired Debt | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	80	[1],[13]	74	[1],[13]
Recoverable Costs Contaminated Facilities
Rate And Regulatory Matters [Line Items]
Regulatory assets	251	[14]	271	[14]
Recoverable Costs Contaminated Facilities | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	251	[14]	271	[14]
Storm Costs
Rate And Regulatory Matters [Line Items]
Regulatory assets	3	[15]	8	[15]
Storm Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets			5	[15]
Storm Costs | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	3	[15]	3	[15]
Demand-Side Costs
Rate And Regulatory Matters [Line Items]
Regulatory assets	44	[1],[16]	58	[1],[16]
Demand-Side Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	44	[1],[16]	58	[1],[16]
Reserve For Workers' Compensation Liabilities
Rate And Regulatory Matters [Line Items]
Regulatory assets	14	[17]	12	[17]
Reserve For Workers' Compensation Liabilities | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	7	[17]	6	[17]
Reserve For Workers' Compensation Liabilities | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	7	[17]	6	[17]
Credit Facilities Fees
Rate And Regulatory Matters [Line Items]
Regulatory assets	5	[18]	5	[18]
Credit Facilities Fees | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	5	[18]	5	[18]
Common Stock Issuance Costs
Rate And Regulatory Matters [Line Items]
Regulatory assets	2	[19]	4	[19]
Common Stock Issuance Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	2	[19]	4	[19]
Construction Accounting For Pollution Control Equipment
Rate And Regulatory Matters [Line Items]
Regulatory assets	21	[1],[20]	22	[1],[20]
Construction Accounting For Pollution Control Equipment | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	21	[1],[20]	22	[1],[20]
Solar Rebates
Rate And Regulatory Matters [Line Items]
Regulatory assets	88	[1],[21]	27	[1],[21]
Solar Rebates | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	88	[1],[21]	27	[1],[21]
Other
Rate And Regulatory Matters [Line Items]
Regulatory assets	11	[22]	20	[22]
Regulatory liabilities	7	[23]	11	[23]
Other | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	5	[22]	8	[22]
Regulatory liabilities	7	[23]	11	[23]
Other | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	6	[22]	12	[22]
Over-Recovered FAC
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	0	[2]	26	[2]
Over-Recovered FAC | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	0	[2]	26	[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	26	[3]	51	[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	26	[3]	51	[3]
Over-Recovered PGA
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	27	[3]	34	[3]
Over-Recovered PGA | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	2	[3]	5	[3]
Over-Recovered PGA | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	25	[3]	29	[3]
MTM Derivative Gains
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	17	[4]	27	[4]
MTM Derivative Gains | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	16	[4]	26	[4]
MTM Derivative Gains | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	1	[4]	1	[4]
Wholesale Distribution Refund
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities			13	[24]
Wholesale Distribution Refund | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities			13	[24]
Refund Reserves for FERC Orders and Audit Findings [Member]
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	25	[25]
Refund Reserves for FERC Orders and Audit Findings [Member] | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	21	[25]
Uncertain tax positions tracker [Member]
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	7	[26]	1	[26]
Uncertain tax positions tracker [Member] | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	7	[26]	1	[26]
Removal Costs
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	1,529	[27]	1,438	[27]
Removal Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	886	[27]	828	[27]
Removal Costs | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	643	[27]	610	[27]
Bad Debt Rider
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	7	[28]	8	[28]
Bad Debt Rider | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	7	[28]	8	[28]
Pension And Postretirement Benefit Costs Tracker
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	24	[29]	15	[29]
Pension And Postretirement Benefit Costs Tracker | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	$ 24	[29]	$ 15	[29]

[1] (a)These assets earn a return.

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

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

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

[6] (f)The difference between Ameren Illinois' annual revenue requirement calculated under the IEIMA's performance-based formula ratemaking framework and the revenue requirement included in customer rates for that year. Subject to ICC approval, these amounts will be collected from or refunded to customers within two years.

[7] (g)Ameren Illinois' and ATXI's annual revenue requirement reconciliation adjustments calculated pursuant to the FERC's electric transmission formula ratemaking framework. The under-recovery or over-recovery will be recovered from or refunded to customers within two years.

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

[9] (i)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.

[10] (x)Unamortized portion of investment tax credits and federal excess deferred taxes. The unamortized portion of investment tax credits and the federal excess deferred taxes are being amortized over the expected life of the underlying assets.

[11] (j)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.

[12] (k)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.

[13] (l)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.

[14] (m)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.

[15] (n)Ameren Missouri's actual storm costs that exceed the normalized storm costs for rate purposes. As approved by the December 2012 MoPSC electric rate order, the 2006, 2007, and 2008 storm costs were amortized through December 2014. The Ameren Illinois balance includes 2013 storm costs deferred in accordance with the IEIMA. These costs are being amortized over a five-year period beginning in 2013.

[16] (o)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.The amortization period for costs incurred from August 2012 through December 2012 will be determined in the July 2014 electric rate case.

[17] (p)The period of recovery will depend on the timing of actual expenditures.

[18] (q)Ameren Missouri’s costs incurred to enter into and maintain the 2012 Missouri Credit Agreement. Additional costs were incurred in December 2014 to amend and restate the 2012 Missouri Credit Agreement. These costs are being amortized over the life of the credit facility, ending in December 2019, to construction work in progress, which will be depreciated when assets are placed into service.

[19] (r)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.

[20] (s)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 was included in customer rates. These costs will be amortized over the expected life of the Sioux energy center, which is currently through 2033.

[21] (t)Costs associated with Ameren Missouri's solar rebate program beginning in August 2012 to fulfill its renewable energy portfolio requirement. The amortization period for these costs will be three years, commencing with the effectiveness of Ameren Missouri's current July 2014 electric rate case.

[22] (u)The Ameren Illinois balance includes Ameren Illinois Merger integration and optimization costs, which are being 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. 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.

[23] (ac)Balance 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 balance also includes a regulatory tracking mechanism at Ameren Missouri for the difference between the level of storm costs incurred in a particular year and the level of such costs built into rates. The amortization periods for these over-recoveries will be determined in the July 2014 electric rate case.

[24] (v)Estimated refund to wholesale electric customers as of December 31, 2013. See 2011 Wholesale Distribution Rate Case above.

[25] (w)Estimated refunds to transmission customers related to FERC orders and audit findings. In regards to the FERC orders, see Ameren Illinois Electric Transmission Rate Refund and FERC Complaint Cases above.

[26] (y)The tracker is amortized over three years, beginning from the date the amounts are included in rates. See Note 13 - Income Taxes for additional information.

[27] (z)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.

[28] (aa)A regulatory tracking mechanism for the difference between the level of bad debt incurred by Ameren Illinois under GAAP and the level of such costs included in electric and natural gas rates. The over-recovery relating to 2012 was refunded to customers from June 2013 through May 2014. The over-recovery relating to 2013 is being refunded to customers from June 2014 through May 2015. The over-recovery relating to 2014 will be refunded to customers from June 2015 through May 2016.

[29] (ab)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 the July 2014 electric rate case.