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Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Financial Assets and Liabilities) (Detail) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	$ 267	$ 301	$ 163	$ 176
Short-term debt	431	490
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	17	145	11	39
Advances to money pool	92	139
Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	242	147	$ 142	$ 125
Advances to money pool	20	0
Borrowings from money pool	0	19
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	267	301
Available-for-sale Securities and Held-to-maturity Securities	256	256
Short-term debt	431	490
Long-term debt (including current portion)	12,500	11,086
Debt Issuance Costs, Net	93	84
Carrying Amount \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	17	145
Advances to money pool	92	139
Available-for-sale Securities and Held-to-maturity Securities	256	256
Long-term debt (including current portion)	5,626	5,104
Debt Issuance Costs, Net	40	36
Carrying Amount \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	242	147
Advances to money pool	20
Long-term debt (including current portion)	4,391	3,946
Debt Issuance Costs, Net	40	36
Borrowings from money pool		19
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	267	301
Investments, Fair Value Disclosure	256	256
Short-term Debt, Fair Value	431	490
Long-term Debt, Fair Value	14,175	13,315
Fair Value \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	267	301
Investments, Fair Value Disclosure	0	0
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Fair Value \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	0	0
Investments, Fair Value Disclosure	256	256
Short-term Debt, Fair Value	431	490
Long-term Debt, Fair Value	13,647	12,778
Fair Value \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
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	528	537
Fair Value \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	17	145
Advances to money pool	92	139
Investments, Fair Value Disclosure	256	256
Long-term Debt, Fair Value	6,427	6,160
Fair Value \| Union Electric Company \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	17	145
Advances to money pool	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Fair Value \| Union Electric Company \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	0	0
Advances to money pool	92	139
Investments, Fair Value Disclosure	256	256
Long-term Debt, Fair Value	6,427	6,160
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
Advances to money pool	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Fair Value \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	242	147
Advances to money pool	20
Long-term Debt, Fair Value	5,038	4,822
Borrowings from money pool		19
Fair Value \| Ameren Illinois Company \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	242	147
Advances to money pool	0
Long-term Debt, Fair Value	0	0
Borrowings from money pool		0
Fair Value \| Ameren Illinois Company \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, cash equivalents, and restricted cash	0	0
Advances to money pool	20
Long-term Debt, Fair Value	5,038	4,822
Borrowings from money pool		19
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
Advances to money pool	0
Long-term Debt, Fair Value	$ 0	0
Borrowings from money pool		$ 0