Related Party Transactions (Narrative) (Details) - Ameren Missouri Software Licensing with Ameren Illinois $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($)
Ameren Illinois Electric Distribution
Related Party Transaction [Line Items]
Revenue from Related Parties	$ 14
Ameren Illinois Gas
Related Party Transaction [Line Items]
Revenue from Related Parties	5
Ameren Illinois Company
Related Party Transaction [Line Items]
Related Party Transaction, Amounts of Transaction	24
Capitalized Computer Software, Net	5
Revenue from Related Parties	19
Union Electric Company
Related Party Transaction [Line Items]
Capitalized Computer Software, Gross	$ 24

Amount before accumulated amortization of capitalized costs for computer software, including but not limited to, acquired and internally developed computer software.

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The carrying amount of capitalized computer software costs net of accumulated amortization as of the balance sheet date.

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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.

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