Rate And Regulatory Matters (Narrative-Missouri) (Detail) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Jan. 31, 2021 MWh	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) MWh	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Rate And Regulatory Matters [Line Items]
Regulatory assets		$ 1,272		$ 1,272		$ 1,100
Revenues		1,472	$ 1,398	3,038	$ 2,838
Union Electric Company
Rate And Regulatory Matters [Line Items]
Atchison Renewable Energy Center purchase price				500
Regulatory assets		430		430		$ 347
Pending Rate Case | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount				$ 299
Public Utilities, Requested Return on Equity, Percentage				9.90%
Public Utilities, Requested Equity Capital Structure, Percentage				51.90%
Rate Base				$ 10,000
Months to complete a rate proceeding				11 months
Pending Rate Case | Natural Gas | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount				$ 9
Public Utilities, Requested Return on Equity, Percentage				9.80%
Public Utilities, Requested Equity Capital Structure, Percentage				51.90%
Rate Base				$ 310
Months to complete a rate proceeding				11 months
Wind Generation Facility | Union Electric Company
Rate And Regulatory Matters [Line Items]
Amount of Megawatts | MWh	300			700
Final Rate Order | Union Electric Company
Rate And Regulatory Matters [Line Items]
COVID-19 pandemic costs				$ 9
Regulatory assets		$ 5		5
Revenues				$ 4