Rate And Regulatory Matters (Schedule Of Regulatory Assets And Liabilities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 1,061	$ 1,261
Current regulatory assets	(69)	(134)
Regulatory Assets, Noncurrent	992	1,127
Regulatory Liabilities	5,051	4,786
Current regulatory liabilities	(164)	(149)
Regulatory Liability, Noncurrent	4,887	4,637
Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	293	380
Current regulatory assets	(8)	(14)
Regulatory Assets, Noncurrent	285	366
Regulatory Liabilities	2,999	2,867
Current regulatory liabilities	(62)	(68)
Regulatory Liability, Noncurrent	$ 2,937	2,799
PISA Deferral Percentage	85.00%
Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 751	869
Current regulatory assets	(57)	(110)
Regulatory Assets, Noncurrent	694	759
Regulatory Liabilities	1,933	1,803
Current regulatory liabilities	(84)	(62)
Regulatory Liability, Noncurrent	1,849	1,741
Under-recovered Illinois electric power costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	4	0
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	0
Amortization Period	1 year
Under-recovered PGA
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 0	7
Under-recovered PGA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	0
Under-recovered PGA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	7
MTM derivative losses
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	254	216
MTM derivative losses | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	12	19
MTM derivative losses | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	242	197
IEIMA revenue requirement reconciliation adjustment
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	17	70
Regulatory Liabilities	18
IEIMA revenue requirement reconciliation adjustment | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	17	70
Regulatory Liabilities	$ 18
Amortization Period	2 years
FERC revenue requirement reconciliation adjustment
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 16	30
Regulatory Liabilities	$ 38	19
Amortization Period	2 years
FERC revenue requirement reconciliation adjustment | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 1	16
Regulatory Liabilities	37	17
Pension and postretirement benefit costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	33	252
Pension and postretirement benefit costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	7	103
Pension and postretirement benefit costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	26	149
Income taxes
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	177	185
Regulatory Liabilities	$ 2,326	2,413
Weighted-Average Amortization Period	34 years
Income taxes | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 114	119
Regulatory Liabilities	$ 1,428	1,484
Weighted-Average Amortization Period	26 years
Income taxes | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 61	68
Regulatory Liabilities	$ 813	843
Weighted-Average Amortization Period	43 years
Callaway costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 18	22
Callaway costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	18	22
Unamortized loss on reacquired debt
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	86	98
Unamortized loss on reacquired debt | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	55	58
Unamortized loss on reacquired debt | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	31	40
Environmental cost riders
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	127	148
Environmental cost riders | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	127	148
Storm costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	7	13
Storm costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	7	13
Workers' compensation claims
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	11	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	7	7
Construction accounting for pollution control equipment
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	15	16
Construction accounting for pollution control equipment | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	15	16
Solar rebate program
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	5	14
Solar rebate program | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	5	14
PISA
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	41	1
PISA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 41	1
Amortization Period	20 years
RESRAM
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 9
RESRAM | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	9
FEJA energy-efficiency rider
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	211	136
FEJA energy-efficiency rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	211	136
Other regulatory assets
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	30	42
Other regulatory assets | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	13	24
Other regulatory assets | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	17	18
Over-recovered FAC
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	39	34
Over-recovered FAC | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 39	34
Amortization Period	8 months
Accumulation Period	4 months
Over-recovered Illinois electric power costs
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 11	12
Over-recovered Illinois electric power costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	11	12
Over-recovered PGA
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	22	10
Over-recovered PGA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	8	7
Over-recovered PGA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	14	3
VBA rider
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	8	8
VBA rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	8	8
MTM derivative gains
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	21	8
MTM derivative gains | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	18	5
MTM derivative gains | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	3	3
MEEIA energy-efficiency rider
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	3	19
MEEIA energy-efficiency rider | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 3	19
Amortization Period	1 year
MEEIA energy-efficiency rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 0	0
Estimated refund for FERC complaint cases
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	40	44
Estimated refund for FERC complaint cases | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	23	26
Cost of removal
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	1,884	1,811
Cost of removal | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	1,041	1,027
Cost of removal | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	827	774
AROs
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	303	175
AROs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	303	175
Pension and postretirement benefit costs tracker
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	72	43
Pension and postretirement benefit costs tracker | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 72	43
Regulatory Liability, Amortization Period	3 years
Renewable energy credits and zero emission credits
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 155	102
Renewable energy credits and zero emission credits | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	155	102
Excess income taxes collected in 2018
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	60	60
Excess income taxes collected in 2018 | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	60	60
Other regulatory liabilities
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	51	28
Other regulatory liabilities | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	27	13
Other regulatory liabilities | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 24	$ 15
FAC adjustments | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Accumulation Period	4 months
Minimum
Public Utilities, General Disclosures [Line Items]
Regulatory Liability, Amortization Period	25 years
Minimum | FEJA energy-efficiency rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Asset, Amortization Period	7 years
Maximum
Public Utilities, General Disclosures [Line Items]
Regulatory Liability, Amortization Period	65 years
Maximum | FEJA energy-efficiency rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Asset, Amortization Period	12 years