Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including nuclear fuel expenditures	$ 503	$ 412
Net realized and unrealized gain – nuclear decommissioning trust fund	91	136
Issuance of common stock for stock-based compensation
Supplemental Cash Flow Information [Line Items]
Stock Issued	25	25
Issuance of common stock under the DRPlus
Supplemental Cash Flow Information [Line Items]
Stock Issued	6	7
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including nuclear fuel expenditures	312	277
Net realized and unrealized gain – nuclear decommissioning trust fund	91	136
Union Electric Company \| Issuance of common stock for stock-based compensation
Supplemental Cash Flow Information [Line Items]
Stock Issued	0	0
Union Electric Company \| Issuance of common stock under the DRPlus
Supplemental Cash Flow Information [Line Items]
Stock Issued	0	0
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including nuclear fuel expenditures	148	147
Net realized and unrealized gain – nuclear decommissioning trust fund	0	0
Ameren Illinois Company \| Issuance of common stock for stock-based compensation
Supplemental Cash Flow Information [Line Items]
Stock Issued	0	0
Ameren Illinois Company \| Issuance of common stock under the DRPlus
Supplemental Cash Flow Information [Line Items]
Stock Issued	$ 0	$ 0

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+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
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+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(9)(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2

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-Topic 220
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-Paragraph 2
-Subparagraph (SX 210.5-03(7)(c))
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-URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2

Reference 3: http://www.xbrl.org/2003/role/recommendedDisclosureRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-Publisher FASB
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