Related Party Transactions (Effects of Related-party Transactions on the Statement of Income (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Union Electric Company \| Ameren Missouri Power Supply Agreements with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues	$ 11	$ 3	$ 11
Union Electric Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	26	27	22
Operating Expenses	1	2	3
Union Electric Company \| Ameren Missouri and Ameren Illinois miscellaneous support services and services provided to ATXI
Related Party Transaction [Line Items]
Operating Revenues	3	1	1
Union Electric Company \| Total Operating Revenues
Related Party Transaction [Line Items]
Operating Revenues	40	31	34
Union Electric Company \| Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	140	135	136
Union Electric Company \| Total Related Party Other Operations and Maintenance
Related Party Transaction [Line Items]
Operating Expenses	140	137	139
Union Electric Company \| Money Pool Borrowings (Advances)
Related Party Transaction [Line Items]
Interest (Charges) Income	1	1	1
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	1	2	3
Operating Expenses	4	5	6
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois miscellaneous support services and services provided to ATXI
Related Party Transaction [Line Items]
Operating Revenues	1	2	1
Ameren Illinois Company \| Ameren Missouri Software Licensing with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues		19
Ameren Illinois Company \| Total Operating Revenues
Related Party Transaction [Line Items]
Operating Revenues	2	23	4
Ameren Illinois Company \| Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Operating Expenses	11	3	11
Ameren Illinois Company \| Ameren Illinois Transmission Services From ATXI
Related Party Transaction [Line Items]
Operating Expenses	2	2	1
Ameren Illinois Company \| Total Purchased Power
Related Party Transaction [Line Items]
Operating Expenses	13	5	12
Ameren Illinois Company \| Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	133	127	126
Ameren Illinois Company \| Total Related Party Other Operations and Maintenance
Related Party Transaction [Line Items]
Operating Expenses	137	132	132
Ameren Illinois Company \| Money Pool Borrowings (Advances)
Related Party Transaction [Line Items]
Interest (Charges) Income	$ 1	$ 1	$ 1

Amount of interest expense incurred on a debt or other obligation to related party.

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- Definition

Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07.1(c))
-URI http://asc.fasb.org/extlink&oid=120401555&loc=SL114874292-224272

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

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