Related Party Transactions Narrative (Details) - April 2018 Procurement [Member] - Ameren Illinois Company - Ameren Illinois Power Supply Agreements with Ameren Missouri	3 Months Ended	6 Months Ended
Jun. 30, 2018 $ / MWh	Jun. 30, 2018 MWh
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power \| MWh		110,000
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate \| $ / MWh	32

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