Rate And Regulatory Matters (Schedule Of Regulatory Assets And Liabilities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Feb. 28, 2021	Dec. 31, 2020
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 1,608		$ 1,209
Current regulatory assets	(319)		(109)
Regulatory Assets, Noncurrent	1,289		1,100
Regulatory Liabilities	5,961		5,403
Current regulatory liabilities	(113)		(121)
Regulatory Liability, Noncurrent	5,848		5,282
Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	650		407
Current regulatory assets	(127)		(60)
Regulatory Assets, Noncurrent	523		347
Regulatory Liabilities	3,411		3,136
Current regulatory liabilities	(57)		(26)
Regulatory Liability, Noncurrent	$ 3,354		3,110
PISA Deferral Percentage	85.00%
Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 932		779
Current regulatory assets	(180)		(37)
Regulatory Assets, Noncurrent	752		742
Regulatory Liabilities	2,428		2,151
Current regulatory liabilities	(54)		(88)
Regulatory Liability, Noncurrent	2,374		2,063
Under-recovered FAC
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	47		48
Under-recovered FAC | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	47		48
Under-recovered FAC | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0		0
Under Recovered PGA
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	163		2
Under Recovered PGA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	49	$ 53	0
Regulatory Assets, Noncurrent		53
Under Recovered PGA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 114	$ 221	2
Regulatory Asset, Amortization Period	18 months
MTM derivative losses
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 202		221
MTM derivative losses | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	77		21
MTM derivative losses | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	125		200
IEIMA revenue requirement reconciliation adjustment
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	42		0
Regulatory Liabilities	0		22
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	42		0
Regulatory Liabilities	$ 0		22
Amortization Period	2 years
FERC revenue requirement reconciliation adjustment
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 43		50
Regulatory Liabilities	$ 4		21
Amortization Period	2 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	18		28
Regulatory Liabilities	2		21
Under/Over-recovered VBA
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	17		11
Under/Over-recovered VBA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0		0
Under/Over-recovered VBA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	17		11
Income taxes
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	185		183
Regulatory Liabilities	$ 2,066		2,192
Weighted-Average Amortization Period	35 years
Income taxes | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 115		117
Regulatory Liabilities	$ 1,208		1,317
Weighted-Average Amortization Period	28 years
Income taxes | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 69		65
Regulatory Liabilities	$ 770		790
Weighted-Average Amortization Period	42 years
Bad debt rider
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 8		11
Regulatory Liabilities	19		5
Bad debt rider | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0		0
Regulatory Liabilities	0		0
Bad debt rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	8		11
Regulatory Liabilities	19		5
Callaway refueling and maintenance outage costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	14		39
Callaway refueling and maintenance outage costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 14		39
Regulatory Asset, Amortization Period	18 months
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	63		74
Unamortized loss on reacquired debt | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	50		52
Unamortized loss on reacquired debt | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	13		22
Environmental cost riders
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	70		93
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	70		93
Storm costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	17		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	17		7
Allowance for funds used during construction for pollution control equipment
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	13		15
Allowance for funds used during construction for pollution control equipment | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	13		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	47		17
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	47		17
PISA
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	244		78
PISA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 244		78
Amortization Period	20 years
Regulatory Asset, Amortization Period	20 years
PISA | 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	350		283
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	350		283
Other regulatory assets
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	83		77
Other regulatory assets | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	41		37
Other regulatory assets | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	42		40
Over-recovered FAC
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	19		10
Over-recovered FAC | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 19		10
Accumulation Period	4 months
Amortization Period	8 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	13		15
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	$ 13		15
Amortization Period	1 year
Over-recovered PGA
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 1		22
Over-recovered PGA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	0		7
Over-recovered PGA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	1		15
MTM derivative gains
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	91		21
MTM derivative gains | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	50		11
MTM derivative gains | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	41		10
Cost of removal
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	1,988		1,923
Cost of removal | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	1,028		1,027
Cost of removal | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	929		873
AROs
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	603		436
AROs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	603		436
AROs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	0		0
Pension and postretirement benefit costs
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	791		375
Pension and postretirement benefit costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	399		198
Pension and postretirement benefit costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	392		177
Pension and postretirement benefit costs tracker
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	28		55
Pension and postretirement benefit costs tracker | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 28		55
Regulatory Liability, Amortization Period	5 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	246		200
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	246		200
RESRAM
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	19		2
RESRAM | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	19		2
RESRAM | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	0		0
Excess income taxes collected in 2018
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	25		45
Excess income taxes collected in 2018 | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	25		45
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	48		59
Other regulatory liabilities | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	32		28
Other regulatory liabilities | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 15		23
FAC adjustments | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Accumulation Period	4 months
Asset Retirement Obligation - Nuclear decommissioning trust fund balance | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 1,159		982
Asset Retirement Obligation - removal costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 556		$ 546
Minimum | Under Recovered PGA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Amortization Period	12 months
Regulatory Asset, Amortization Period	12 months
Minimum | FEJA energy-efficiency rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Asset, Amortization Period	5 years
Maximum | Under Recovered PGA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Amortization Period	36 months
Regulatory Asset, Amortization Period	36 months
Maximum | FEJA energy-efficiency rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Asset, Amortization Period	13 years