Rate And Regulatory Matters (Narrative-Missouri) (Detail) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) MWh	Mar. 31, 2019 USD ($)	Dec. 31, 2019 MWh	Dec. 31, 2018 MWh
Rate And Regulatory Matters [Line Items]
Purchased Power	$ 134	$ 156
Other operations and maintenance	438	417
Income Taxes	21	27
Donations	13	6
Union Electric Company
Rate And Regulatory Matters [Line Items]
Purchased Power	39	51
Other operations and maintenance	239	224
Income Taxes	(1)	4
Donations	8	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
PISA deferrals	70
Donations	$ 8
Wind Generation Facility | Union Electric Company
Rate And Regulatory Matters [Line Items]
Amount of Megawatts | MWh	300		300	400
Project Placed In-Service Amount	$ 100
Estimated Capital Project Costs	$ 1,200
MEEIA 2013 & 2016 | Final Rate Order | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Revenues		$ 20
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%