Rate And Regulatory Matters (Narrative-Missouri) (Detail) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 MWh	Dec. 31, 2018 MWh
Rate And Regulatory Matters [Line Items]
Purchased Power		$ 109	$ 136	$ 243	$ 292
Other operations and maintenance		384	450	822	867
Income Taxes		50	39	71	66
Donations		1	1	14	7
Union Electric Company
Rate And Regulatory Matters [Line Items]
Purchased Power		37	60	76	111
Other operations and maintenance		202	254	441	478
Income Taxes		12	15	11	19
Donations		1	$ 0	9	$ 2
Final Rate Order | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount				32
Reduction to annualized base level of net energy costs				115
Revenues				20
Purchased Power				15
Other operations and maintenance				60
Income Taxes				20
Revenue requirement change due to MEEIA sales volumes				50
Amounts previously deferred under PISA				50
Donations		$ 8		8
Wind Generation Facility | Union Electric Company
Rate And Regulatory Matters [Line Items]
Amount of Megawatts | MWh						300	400
Estimated Capital Project Costs				1,200
Project Placed In-Service Amount				100
MEEIA 2013 & 2016 | Final Rate Order | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Revenues				20
MEEIA 2019 [Member] | Final Rate Order | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved				11
Incentive Award if Energy Efficiency Goals Are Exceeded				1
MEEIA Energy Efficiency Investments for the 2022 Program Year				$ 70
MEEIA 2019 [Member] | Final Rate Order | Electricity | Union Electric Company | Subsequent Event
Rate And Regulatory Matters [Line Items]
Revenues	$ 6
Minimum | Final Rate Order | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage				9.40%
Maximum | Final Rate Order | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage				9.80%