Rate And Regulatory Matters (Schedule Of Regulatory Assets And Liabilities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 1,209	$ 1,061
Current regulatory assets	(109)	(69)
Regulatory Assets, Noncurrent	1,100	992
Regulatory Liabilities	5,403	5,051
Current regulatory liabilities	(121)	(164)
Regulatory Liability, Noncurrent	5,282	4,887
Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	407	293
Current regulatory assets	(60)	(8)
Regulatory Assets, Noncurrent	347	285
Regulatory Liabilities	3,136	2,999
Current regulatory liabilities	(26)	(62)
Regulatory Liability, Noncurrent	$ 3,110	2,937
Period between refueling and maintenance outages	18 months
PISA Deferral Percentage	85.00%
Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 779	751
Current regulatory assets	(37)	(57)
Regulatory Assets, Noncurrent	742	694
Regulatory Liabilities	2,151	1,933
Current regulatory liabilities	(88)	(84)
Regulatory Liability, Noncurrent	2,063	1,849
Under-recovered FAC
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	48	0
Under-recovered FAC | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	48	0
Under-recovered FAC | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Under-recovered Illinois electric power costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	4	4
Under-recovered Illinois electric power costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Under-recovered Illinois electric power costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 4	4
Amortization Period	one year
MTM derivative losses
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 221	254
MTM derivative losses | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	21	12
MTM derivative losses | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	200	242
IEIMA revenue requirement reconciliation adjustment
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	17
Regulatory Liabilities	22	18
IEIMA revenue requirement reconciliation adjustment | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Regulatory Liabilities	0	0
IEIMA revenue requirement reconciliation adjustment | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	17
Regulatory Liabilities	$ 22	18
Amortization Period	two years
FERC revenue requirement reconciliation adjustment
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 50	16
Regulatory Liabilities	$ 21	38
Amortization Period	two years
FERC revenue requirement reconciliation adjustment | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 0	0
Regulatory Liabilities	0	0
FERC revenue requirement reconciliation adjustment | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	28	1
Regulatory Liabilities	21	37
Under/Over-recovered VBA
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	11	0
Regulatory Liabilities	0	8
Under/Over-recovered VBA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Regulatory Liabilities	0	0
Under/Over-recovered VBA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	11	0
Regulatory Liabilities	0	8
Pension and postretirement benefit costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	33
Regulatory Liabilities	375	0
Pension and postretirement benefit costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	7
Regulatory Liabilities	198	0
Pension and postretirement benefit costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	26
Regulatory Liabilities	177	0
Income taxes
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	183	177
Regulatory Liabilities	$ 2,192	2,326
Weighted-Average Amortization Period	34 years
Income taxes | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 117	114
Regulatory Liabilities	$ 1,317	1,428
Weighted-Average Amortization Period	26 years
Income taxes | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 65	61
Regulatory Liabilities	$ 790	813
Weighted-Average Amortization Period	42 years
Bad debt rider
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 11	0
Bad debt rider | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Bad debt rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	11	0
Callaway costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	14	18
Callaway costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	14	18
Callaway costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Callaway refueling and maintenance outage costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	39	0
Callaway refueling and maintenance outage costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	39	0
Callaway refueling and maintenance outage costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Unamortized loss on reacquired debt
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	74	86
Unamortized loss on reacquired debt | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	52	55
Unamortized loss on reacquired debt | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	22	31
Environmental cost riders
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	93	127
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	93	127
Storm costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	7	7
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	7	7
Workers' compensation claims
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	9	11
Workers' compensation claims | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	4	4
Workers' compensation claims | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	5	7
Allowance for funds used during construction for pollution control equipment
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	15	15
Allowance for funds used during construction for pollution control equipment | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	15	15
Allowance for funds used during construction for pollution control equipment | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Customer generation rebate program
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	17	1
Customer generation rebate program | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Customer generation rebate program | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	17	1
Solar rebate program
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	5	5
Solar rebate program | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	5	5
Solar rebate program | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
PISA
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	78	41
PISA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 78	41
Amortization Period	20 years
PISA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 0	0
RESRAM
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	2	9
RESRAM | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	2	9
RESRAM | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
FEJA energy-efficiency rider
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	283	211
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	283	211
Other regulatory assets
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	45	29
Other regulatory assets | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	12	13
Other regulatory assets | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	33	16
Over-recovered FAC
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	10	39
Over-recovered FAC | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 10	39
Accumulation Period	four months
Amortization Period	eight months
Over-recovered FAC | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 0	0
Over-recovered Illinois electric power costs
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	15	11
Over-recovered Illinois electric power costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	0	0
Over-recovered Illinois electric power costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	15	11
Over-recovered PGA
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	22	22
Over-recovered PGA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	7	8
Over-recovered PGA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	15	14
MTM derivative gains
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	21	21
MTM derivative gains | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	11	18
MTM derivative gains | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	10	3
Estimated refund for FERC complaint cases
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	15	40
Estimated refund for FERC complaint cases | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	0	0
Estimated refund for FERC complaint cases | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	7	23
Cost of removal
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	1,923	1,884
Cost of removal | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	1,027	1,041
Cost of removal | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	873	827
AROs
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	436	303
AROs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	436	303
AROs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	0	0
Pension and postretirement benefit costs tracker
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	55	72
Pension and postretirement benefit costs tracker | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 55	72
Regulatory Liability, Amortization Period	4 years
Pension and postretirement benefit costs tracker | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 0	0
Renewable energy credits and zero emission credits
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	200	155
Renewable energy credits and zero emission credits | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	0	0
Renewable energy credits and zero emission credits | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	200	155
Excess income taxes collected in 2018
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	45	60
Excess income taxes collected in 2018 | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	45	60
Excess income taxes collected in 2018 | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	0	0
Other regulatory liabilities
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	51	54
Other regulatory liabilities | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	30	30
Other regulatory liabilities | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 21	$ 24
FAC adjustments | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Accumulation Period	four months
Asset Retirement Obligation - Nuclear decommissioning trust fund balance | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 982
Asset Retirement Obligation - removal costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 546
Minimum | FEJA energy-efficiency rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Asset, Amortization Period	6 years
Maximum | FEJA energy-efficiency rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Asset, Amortization Period	13 years