Rate And Regulatory Matters (Narrative-Missouri) (Detail) $ in Millions	3 Months Ended			7 Months Ended	9 Months Ended		12 Months Ended
	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($) MWh	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($) MWh	Sep. 30, 2019 USD ($)	Dec. 31, 2019 MWh	Dec. 31, 2018 MWh
Rate And Regulatory Matters [Line Items]
Purchased Power		$ 140	$ 148		$ 383	$ 440
Other operations and maintenance		418	434		1,240	1,301
Income Taxes		63	92		134	158
Donations		0	1		14	8
Union Electric Company
Rate And Regulatory Matters [Line Items]
Purchased Power		48	49		124	160
Other operations and maintenance		221	242		662	720
Income Taxes		18	51		29	70
Donations		$ 0	$ 1		9	3
Net costs and forgone late fees related to the COVID-19 Pandemic				$ 9
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
Wind Generation Facility | Union Electric Company
Rate And Regulatory Matters [Line Items]
Amount of Megawatts | MWh		400			300		300	400
Estimated Capital Project Costs					$ 1,200
Wind Generation Facility | Union Electric Company | Subsequent Event
Rate And Regulatory Matters [Line Items]
Project Placed In-Service Amount	$ 200
MEEIA 2019 [Member] | Final Rate Order | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Revenues		$ 6
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 2013 & 2016 | Final Rate Order | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Revenues						$ 38
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%