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Related Party Transactions Narrative (Details) - April 2018 Procurement - Ameren Illinois Company - Ameren Illinois Power Supply Agreements with Ameren Missouri
3 Months Ended 9 Months Ended
Sep. 30, 2018
$ / MWh
Sep. 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