Related Party Transactions (Narrative) (Details) - Ameren Illinois Company - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019
Related Party Transaction [Line Items]
Capital contribution from parent	$ 350	$ 0
April 2020 Procurement \| Ameren Illinois Capacity Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Energy Supply Agreements Amount	$ 2

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