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Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Financial Assets and Liabilities) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	$ 246	$ 216	$ 180	$ 155
Short-term debt	1,340	1,070
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	13	13	7	8
Short-term debt	157	329
Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	225	191	$ 166	$ 133
Short-term debt	59	264
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	246	216
Short-term debt	1,340	1,070
Long-term debt (including current portion)	14,678	14,025
Investments in industrial development revenue bonds		240
Debt Issuance Costs, Net	105	99
Carrying Amount \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	13	13
Short-term debt	157	329
Long-term debt (including current portion)	6,341	6,086
Investments in industrial development revenue bonds		240
Debt Issuance Costs, Net	45	41
Carrying Amount \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	225	191
Short-term debt	59	264
Long-term debt (including current portion)	5,231	4,835
Debt Issuance Costs, Net	48	44
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	246	216
Short-term Debt, Fair Value	1,340	1,070
Long-term Debt, Fair Value	12,417	12,453
Investments, Fair Value Disclosure		240
Fair Value \| Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	246	216
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure		0
Fair Value \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	0	0
Short-term Debt, Fair Value	1,340	1,070
Long-term Debt, Fair Value	11,985	11,989
Investments, Fair Value Disclosure		240
Fair Value \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	0	0
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	432	464
Investments, Fair Value Disclosure		0
Fair Value \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	13	13
Short-term Debt, Fair Value	157	329
Long-term Debt, Fair Value	5,287	5,365
Investments, Fair Value Disclosure		240
Fair Value \| Union Electric Company \| Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	13	13
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure		0
Fair Value \| Union Electric Company \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	0	0
Short-term Debt, Fair Value	157	329
Long-term Debt, Fair Value	5,287	5,365
Investments, Fair Value Disclosure		240
Fair Value \| Union Electric Company \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	0	0
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure		0
Fair Value \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	225	191
Short-term Debt, Fair Value	59	264
Long-term Debt, Fair Value	4,419	4,320
Fair Value \| Ameren Illinois Company \| Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	225	191
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Fair Value \| Ameren Illinois Company \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	0	0
Short-term Debt, Fair Value	59	264
Long-term Debt, Fair Value	4,419	4,320
Fair Value \| Ameren Illinois Company \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	0	0
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	$ 0	$ 0