Rate And Regulatory Matters (Narrative-Missouri) (Detail) $ in Millions	1 Months Ended	3 Months Ended
	Jan. 31, 2021 MWh	Mar. 31, 2021 USD ($) MWh	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Rate And Regulatory Matters [Line Items]
Regulatory assets		$ 1,249		$ 1,100
Revenues		1,566	$ 1,440
Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets		413		$ 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
Atchison Renewable Energy Center purchase price		$ 250
Final Rate Order | Union Electric Company
Rate And Regulatory Matters [Line Items]
COVID-19 pandemic costs		9
Regulatory assets		5
Revenues		$ 4