Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	$ 1,152	$ 977
Assets	1,242	999
Excluded receivables, payables, and accrued income, net	7	5
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	824	680
Assets	848	682
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	328	297
Assets	373	306
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets	21	11
Power | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	13	5
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	90	22
Derivative liabilities	202	221
Commodity Contract | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	24	2
Derivative liabilities	45	14
Commodity Contract | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	45	9
Derivative liabilities	7	2
Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	21	11
Derivative liabilities	150	205
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	1,201	989
Union Electric Company | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	847	682
Union Electric Company | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	340	300
Union Electric Company | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	14	7
Union Electric Company | Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	13	2
Derivative liabilities	0	9
Union Electric Company | Fuel Oils | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	13	0
Derivative liabilities	0	6
Union Electric Company | Fuel Oils | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Union Electric Company | Fuel Oils | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	2
Derivative liabilities	0	3
Union Electric Company | Natural gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	12	3
Derivative liabilities	3	1
Union Electric Company | Natural gas | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Union Electric Company | Natural gas | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	12	3
Derivative liabilities	2	1
Union Electric Company | Natural gas | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	1	0
Union Electric Company | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	23	7
Derivative liabilities	73	11
Union Electric Company | Power | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	10	2
Derivative liabilities	45	8
Union Electric Company | Power | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Union Electric Company | Power | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	13	5
Derivative liabilities	28	3
Union Electric Company | Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	0
Derivative liabilities	1	0
Union Electric Company | Uranium | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Union Electric Company | Uranium | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Union Electric Company | Uranium | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	0
Derivative liabilities	1	0
Union Electric Company | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	49	12
Derivative liabilities	77	21
Union Electric Company | Commodity Contract | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	23	2
Derivative liabilities	45	14
Union Electric Company | Commodity Contract | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	12	3
Derivative liabilities	2	1
Union Electric Company | Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	14	7
Derivative liabilities	30	6
Union Electric Company | Equity securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	824	680
Union Electric Company | Equity securities | U.S. large capitalization | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	824	680
Union Electric Company | Equity securities | U.S. large capitalization | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Equity securities | U.S. large capitalization | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt securities | U.S. Treasury and agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	141	115
Union Electric Company | Debt securities | U.S. Treasury and agency securities | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt securities | U.S. Treasury and agency securities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	141	115
Union Electric Company | Debt securities | U.S. Treasury and agency securities | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	131	115
Union Electric Company | Debt securities | Corporate bonds | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt securities | Corporate bonds | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	131	115
Union Electric Company | Debt securities | Corporate bonds | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt securities | Diversified credit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	56	67
Union Electric Company | Debt securities | Diversified credit | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt securities | Diversified credit | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	56	67
Union Electric Company | Debt securities | Diversified credit | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Ameren Illinois Company | Natural gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	41	10
Derivative liabilities	8	2
Ameren Illinois Company | Natural gas | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	0
Derivative liabilities	0	0
Ameren Illinois Company | Natural gas | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	33	6
Derivative liabilities	5	1
Ameren Illinois Company | Natural gas | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	7	4
Derivative liabilities	3	1
Ameren Illinois Company | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	117	198
Ameren Illinois Company | Power | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Power | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Power | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	117	198
Ameren Illinois Company | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	125	200
Ameren Illinois Company | Commodity Contract | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Commodity Contract | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	5	1
Ameren Illinois Company | Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	$ 120	$ 199