Related Party Transactions (Schedule of Related Party Transactions) (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Related Party Transaction [Line Items]
Operating Revenues	$ 0	$ 0
Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	32	33
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	0	0
Operating Expenses	31	32
Ameren Missouri Power Supply Agreements with Ameren Illinois \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1	3
Ameren Missouri and Ameren Illinois Rent and Facility Services \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	7	5
Operating Expenses	1	1
Ameren Missouri and Ameren Illinois Rent and Facility Services \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1
Operating Expenses	1	2
Total Related Party Operating Revenues [Member] \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	7	8
Total Related Party Operating Revenues [Member] \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1
Ameren Missouri and Ameren Illinois miscellaneous support services \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1	1
Ameren Missouri and Ameren Illinois miscellaneous support services \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1
Ameren Illinois Power Supply Agreements with Ameren Missouri \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1	3
Ameren Illinois transmission services from ATXI \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1	1
Purchased Power \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1	3
Ameren Services Support Services Agreement \| Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	32	33
Ameren Services Support Services Agreement \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	30	30
Money Pool [Member] \| Union Electric Company
Related Party Transaction [Line Items]
Interest Charges	0	1
Money Pool [Member] \| Ameren Illinois Company
Related Party Transaction [Line Items]
Interest Charges	$ 0	$ 1

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

No definition available.

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Balance Type:	debit
Period Type:	duration

- 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

+ Details

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Balance Type:	debit
Period Type:	duration

- 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.

+ Details

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- Definition

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=114873854&loc=SL114874292-224272

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(e))
-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

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