Rate And Regulatory Matters (Schedule Of Regulatory Assets And Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	$ 144	$ 149
Regulatory assets	1,230	1,437
Current regulatory liabilities	128	110
Regulatory liabilities	4,387	1,985
Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	56	35
Regulatory assets	395	619
Current regulatory liabilities	19	12
Regulatory liabilities	2,664	1,215
Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	87	108
Regulatory assets	822	816
Current regulatory liabilities	92	78
Regulatory liabilities	1,629	768
Under-Recovered FAC
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	47	21
Under-Recovered FAC | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	47	21
Under-Recovered FAC | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	0	0
Under-Recovered Illinois Electric Power Costs
Public Utilities, General Disclosures [Line Items]
Current regulatory assets		3
Under-Recovered Illinois Electric Power Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	0	0
Under-Recovered Illinois Electric Power Costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets		3
Under-Recovered PGA
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	14	4
Under-Recovered PGA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	1	0
Under-Recovered PGA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	13	4
MTM Derivative Losses
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	33	24
Regulatory assets	196	187
MTM Derivative Losses | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	8	9
Regulatory assets	4	9
MTM Derivative Losses | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	25	15
Regulatory assets	192	178
Energy Efficiency Rider
Public Utilities, General Disclosures [Line Items]
Current regulatory assets		5
Current regulatory liabilities	42	0
Regulatory liabilities		45
Energy Efficiency Rider | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets		5
Current regulatory liabilities	2	0
Regulatory liabilities	0	0
Energy Efficiency Rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	0	0
Current regulatory liabilities	40	0
Regulatory liabilities		45
IEIMA
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	24	68
Regulatory assets	54	23
IEIMA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	0	0
Regulatory assets	0	0
IEIMA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	24	68
Regulatory assets	54	23
FERC Revenue Requirement Reconciliation
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	10	13
Regulatory assets	27	10
FERC Revenue Requirement Reconciliation | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	0	0
Regulatory assets	0	0
FERC Revenue Requirement Reconciliation | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	9	7
Regulatory assets	16	8
VBA Rider
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	15	11
VBA Rider | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	0	0
VBA Rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	15	11
Other Regulatory Assets
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	1	0
Regulatory assets	17	16
Other Regulatory Assets | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	0	0
Regulatory assets	9	9
Other Regulatory Assets | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	1	0
Regulatory assets	8	7
Pension And Postretirement Benefit Costs
Public Utilities, General Disclosures [Line Items]
Regulatory assets	299	494
Pension And Postretirement Benefit Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	84	175
Pension And Postretirement Benefit Costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	215	319
Income Taxes
Public Utilities, General Disclosures [Line Items]
Regulatory assets	197	230
Regulatory liabilities	2,323	37
Income Taxes | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	139	229
Regulatory liabilities	1,392	33
Income Taxes | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	56	1
Regulatory liabilities	842	4
Uncertain tax positions tracker
Public Utilities, General Disclosures [Line Items]
Regulatory assets	5	7
Regulatory liabilities	2	3
Uncertain tax positions tracker | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	5	7
Regulatory liabilities	2	3
Uncertain tax positions tracker | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	0	0
Asset Retirement Obligation
Public Utilities, General Disclosures [Line Items]
Regulatory assets	1	3
Regulatory liabilities	223	162
Asset Retirement Obligation | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	0	0
Regulatory liabilities	223	162
Asset Retirement Obligation | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	1	3
Regulatory liabilities	0	0
Callaway Costs
Public Utilities, General Disclosures [Line Items]
Regulatory assets	25	29
Callaway Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	25	29
Unamortized Loss On Reacquired Debt
Public Utilities, General Disclosures [Line Items]
Regulatory assets	110	124
Unamortized Loss On Reacquired Debt | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	61	65
Unamortized Loss On Reacquired Debt | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	49	59
Environmental cost riders
Public Utilities, General Disclosures [Line Items]
Regulatory assets	173	196
Environmental cost riders | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	0	0
Environmental cost riders | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	173	196
Storm Costs
Public Utilities, General Disclosures [Line Items]
Regulatory assets	10	15
Storm Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	0	0
Storm Costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	10	15
Demand-Side Costs
Public Utilities, General Disclosures [Line Items]
Regulatory assets	11	18
Demand-Side Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	11	18
Reserve For Workers' Compensation Liabilities
Public Utilities, General Disclosures [Line Items]
Regulatory assets	12	13
Reserve For Workers' Compensation Liabilities | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	5	6
Reserve For Workers' Compensation Liabilities | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	7	7
Credit Facilities Fees
Public Utilities, General Disclosures [Line Items]
Regulatory assets	3	4
Credit Facilities Fees | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	3	4
Construction Accounting For Pollution Control Equipment
Public Utilities, General Disclosures [Line Items]
Regulatory assets	18	19
Construction Accounting For Pollution Control Equipment | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	18	19
Solar Rebates
Public Utilities, General Disclosures [Line Items]
Regulatory assets	31	49
Solar Rebates | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	31	49
FEJA energy-efficiency rider
Public Utilities, General Disclosures [Line Items]
Regulatory assets	41	0
FEJA energy-efficiency rider | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	0	0
FEJA energy-efficiency rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	41	0
Over-Recovered FAC
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	4	0
Over-Recovered FAC | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	4	0
Over-Recovered FAC | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	0	0
Over-Recovered Illinois Electric Power Costs
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	16	25
Over-Recovered Illinois Electric Power Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	0	0
Over-Recovered Illinois Electric Power Costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	16	25
Over-Recovered PGA
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	1	0
Over-Recovered PGA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	0	0
Over-Recovered PGA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	1	0
MTM Derivative Gains
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	13	23
MTM Derivative Gains | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	13	12
MTM Derivative Gains | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	0	11
Refund Reserves for FERC Orders and Audit Findings
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	42	62
Refund Reserves for FERC Orders and Audit Findings | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	25	42
Other Regulatory Liabilities
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	10	0
Regulatory liabilities	13	9
Other Regulatory Liabilities | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	0	0
Regulatory liabilities	11	5
Other Regulatory Liabilities | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	10	0
Regulatory liabilities	2	4
Removal Costs
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	1,725	1,669
Removal Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	995	970
Removal Costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	725	697
Bad Debt Rider
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	2	3
Bad Debt Rider | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	0	0
Bad Debt Rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	2	3
Pension And Postretirement Benefit Costs Tracker
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	35	35
Pension And Postretirement Benefit Costs Tracker | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	35	35
Renewable Energy Credits
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	58	15
Renewable Energy Credits | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	0	0
Renewable Energy Credits | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	58	15
Storm Tracker
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	6	7
Storm Tracker | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	6	7
Storm Tracker | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	$ 0	$ 0