Related Party Transactions (Schedule of Capital Contributions) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Union Electric Company
Related Party Transaction [Line Items]
Capital contribution from parent	$ 491	$ 124	$ 45
Ameren Illinois Company
Related Party Transaction [Line Items]
Capital contribution from parent	$ 464	$ 15	$ 160

The cash inflow from parent as a source of financing that is recorded as additional paid in capital.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3255-108585

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration