Derivative Financial Instruments (Offsetting Assets and Liabilities) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 90	$ 22
Derivative Liability, Fair Value, Gross Liability	202	221
Derivative, Collateral, Obligation to Return Securities	19
Derivative, Collateral, Right to Reclaim Securities	19
Derivative Asset, Collateral, Obligation to Return Cash, Offset	0
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	47
Derivative Asset	71
Derivative Liability	136
Union Electric Company
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	49	12
Derivative Liability, Fair Value, Gross Liability	77	21
Derivative, Collateral, Obligation to Return Securities	15
Derivative, Collateral, Right to Reclaim Securities	15
Derivative Asset, Collateral, Obligation to Return Cash, Offset	0
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	47
Derivative Asset	34
Derivative Liability	15
Ameren Illinois Company
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	41	10
Derivative Liability, Fair Value, Gross Liability	125	$ 200
Derivative, Collateral, Obligation to Return Securities	4
Derivative, Collateral, Right to Reclaim Securities	4
Derivative Asset, Collateral, Obligation to Return Cash, Offset	0
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	0
Derivative Asset	37
Derivative Liability	$ 121