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Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Financial Assets and Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash, cash equivalents, and restricted cash	$ 155	$ 301	$ 176	$ 107
Short-term debt	545	490
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash, cash equivalents, and restricted cash	8	145	39	8
Advances to money pool	0	139
Short-term debt	165	0
Debt Issuance Costs, Net	38	36
Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash, cash equivalents, and restricted cash	133	147	$ 125	$ 80
Short-term debt	103	0
Borrowings from money pool	0	19
Debt Issuance Costs, Net	39	36
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash, cash equivalents, and restricted cash	155	301
Investments in industrial development revenue bonds	248	256
Short-term debt	545	490
Long-term debt (including current portion)	13,067	11,086
Debt Issuance Costs, Net	94	84
Carrying Amount \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash, cash equivalents, and restricted cash	8	145
Advances to money pool		139
Investments in industrial development revenue bonds	248	256
Short-term debt	165
Long-term debt (including current portion)	5,619	5,104
Debt Issuance Costs, Net	38	36
Carrying Amount \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash, cash equivalents, and restricted cash	133	147
Short-term debt	103
Long-term debt (including current portion)	4,392	3,946
Borrowings from money pool		19
Debt Issuance Costs, Net	39	36
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash, cash equivalents, and restricted cash	155	301
Investments, Fair Value Disclosure	248	256
Short-term Debt, Fair Value	545	490
Long-term Debt, Fair Value	14,521	13,315
Fair Value \| Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash, cash equivalents, and restricted cash	155	301
Investments, Fair Value Disclosure	0	0
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Fair Value \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash, cash equivalents, and restricted cash	0	0
Investments, Fair Value Disclosure	248	256
Short-term Debt, Fair Value	545	490
Long-term Debt, Fair Value	13,930	12,778
Fair Value \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash, cash equivalents, and restricted cash	0	0
Investments, Fair Value Disclosure	0	0
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	591	537
Fair Value \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash, cash equivalents, and restricted cash	8	145
Advances to money pool		139
Investments, Fair Value Disclosure	248	256
Short-term Debt, Fair Value	165
Long-term Debt, Fair Value	6,321	6,160
Fair Value \| Union Electric Company \| Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash, cash equivalents, and restricted cash	8	145
Advances to money pool		0
Investments, Fair Value Disclosure	0	0
Short-term Debt, Fair Value	0
Long-term Debt, Fair Value	0	0
Fair Value \| Union Electric Company \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash, cash equivalents, and restricted cash	0	0
Advances to money pool		139
Investments, Fair Value Disclosure	248	256
Short-term Debt, Fair Value	165
Long-term Debt, Fair Value	6,321	6,160
Fair Value \| Union Electric Company \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash, cash equivalents, and restricted cash	0	0
Advances to money pool		0
Investments, Fair Value Disclosure	0	0
Short-term Debt, Fair Value	0
Long-term Debt, Fair Value	0	0
Fair Value \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash, cash equivalents, and restricted cash	133	147
Short-term Debt, Fair Value	103
Long-term Debt, Fair Value	4,971	4,822
Borrowings from money pool		19
Fair Value \| Ameren Illinois Company \| Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash, cash equivalents, and restricted cash	133	147
Short-term Debt, Fair Value	0
Long-term Debt, Fair Value	0	0
Borrowings from money pool		0
Fair Value \| Ameren Illinois Company \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash, cash equivalents, and restricted cash	0	0
Short-term Debt, Fair Value	103
Long-term Debt, Fair Value	4,971	4,822
Borrowings from money pool		19
Fair Value \| Ameren Illinois Company \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash, cash equivalents, and restricted cash	0	0
Short-term Debt, Fair Value	0
Long-term Debt, Fair Value	$ 0	0
Borrowings from money pool		$ 0