Supplemental Information (Allowance For Funds Used During Construction) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Allowance for Funds Used During Construction, Rate [Line Items]
Allowance for equity funds used during construction	$ 32	$ 28	$ 36
Allowance for borrowed funds used during construction	16	20	21
Total	$ 48	$ 48	$ 57
Union Electric Company
Allowance for Funds Used During Construction, Rate [Line Items]
Public Utilities, Allowance for Funds Used During Construction, Rate	5.00%	6.00%	7.00%
Allowance for equity funds used during construction	$ 19	$ 19	$ 27
Allowance for borrowed funds used during construction	10	12	14
Total	$ 29	$ 31	$ 41
Ameren Illinois Company
Allowance for Funds Used During Construction, Rate [Line Items]
Public Utilities, Allowance for Funds Used During Construction, Rate	5.00%	5.00%	5.00%
Allowance for equity funds used during construction	$ 13	$ 9	$ 9
Allowance for borrowed funds used during construction	6	8	7
Total	$ 19	$ 17	$ 16

[Line Items] for Allowance for Funds Used During Construction, Rate

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- Definition

Total increase in earnings in the period representing the cost of equity (based on assumed rate of return) and/or borrowed funds (based on interest rate) used to finance construction of regulated assets, which is expected to be recovered through rate adjustments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
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-Section 25
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The component of the allowance for funds used during construction during the period based on an assumed rate of return on equity funds used in financing the construction of regulated assets.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 835
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Amount capitalized of allowance for funds used during construction.

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- Definition

Effective rate, including composite rate, of cost of financing additions to qualifying regulated assets, commonly called allowance for funds used during construction.

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