Callaway Energy Center (Insurance Disclosure) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Nuclear Waste Matters [Line Items]
Decommissioning Cost	$ 7.0
Frequency of Decommissioning Cost Study	3 years
Miscellaneous accounts receivable	$ 111.0	$ 65.0
Number Of Years The Limit Of Liability And The Maximum Potential Annual Payments Are Adjusted	5 years
Number Of Weeks Of Coverage After The First Twelve Weeks Of An Outage	1
Number Of Additional Weeks After Initial Indemnity Coverage For Power Outage	1.365
Union Electric Company
Nuclear Waste Matters [Line Items]
Miscellaneous accounts receivable	$ 93.0	$ 36.0
Nuclear Plant | Union Electric Company
Nuclear Waste Matters [Line Items]
Estimated Nuclear Generator Repairs	60.0
Accidental Outage - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Amount Of Weekly Indemnity Coverage Commencing Twelve Weeks After Power Outage	4.5
Insurance Aggregate Maximum Coverage	490.0
Insurance Maximum Coverage per Incident	7.0
Amount Of Additional Weekly Indemnity Coverage Commencing After Initial Indemnity Coverage	3.6
Amount Of Weekly Indemnity Coverage Thereafter Not Exceeding Policy Limit	490.0
Sub-Limit Of Amount Of Weekly Indemnity Coverage Thereafter Not Exceeding Policy Limit For Non-Nuclear Events	328.0
Public Liability And Nuclear Worker Liability - American Nuclear Insurers
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	450.0
Insurance Maximum Coverage per Incident	0.0
Public Liability And Nuclear Worker Liability - Pool Participation
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	13,073.0
Insurance Maximum Coverage per Incident	138.0
Threshold Amount For Retrospective Insurance Assessment For Covered Loss Under Public Liability And Nuclear Worker Liability Insurance Policy	450.0
Maximum Annual Payment Per Incident At Licensed Commercial Nuclear Reactor	21.0
Public Liability
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	13,523.0
Insurance Maximum Coverage per Incident	138.0
Property Damage - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	3,200.0
Insurance Maximum Coverage per Incident	25.0
Radiation Event
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	2,700.0
Aggregate Nuclear Power Industry Insurance Policy Limit For Losses From Terrorist Attacks Within Twelve Month Period	3,200.0
Non-Radiation Event
Nuclear Waste Matters [Line Items]
Miscellaneous accounts receivable	35.0
Insurance Aggregate Maximum Coverage	2,300.0
Aggregate Nuclear Power Industry Insurance Policy Limit For Losses From Terrorist Attacks Within Twelve Month Period	1,800.0
Property Damage European Mutual Association for Nuclear Insurance
Nuclear Waste Matters [Line Items]
Insurance Aggregate Maximum Coverage	$ 490.0