Rate And Regulatory Matters (Narrative-Missouri) (Detail) $ in Millions	3 Months Ended					9 Months Ended
	Dec. 31, 2019	Sep. 30, 2019 USD ($) MWh	Mar. 31, 2019 USD ($)	Dec. 31, 2018 MWh	Sep. 30, 2018 USD ($) MWh	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)
Rate And Regulatory Matters [Line Items]
Total operating revenues		$ 1,640			$ 1,724	$ 4,575	$ 4,872
Union Electric Company
Rate And Regulatory Matters [Line Items]
Frequency Rates Must be Reset to Use the FAC		four years
Depreciation Percentage Not Included in PISA Deferral						15.00%
Pending Rate Case | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 1
Public Utilities, Requested Return on Equity, Percentage		9.95%
Public Utilities, Requested Equity Capital Structure, Percentage		51.90%
Rate Base		$ 8,000
Number of Months to Complete MoPSC Electric Service Proceeding		11 months
Pending Rate Case | Natural Gas | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 1
Public Utilities, Requested Equity Capital Structure, Percentage		52.00%
ISRS Return on Equity Percentage		9.725%
Public Utilities, Requested Rate Increase (Decrease) on Interim Rates, Amount		$ 1
Components of Rate Decrease - Net Energy Costs | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		100
Components of Rate Decrease - Customer Sales Volume | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		55
Components of Rate Decrease - Other Expenses | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		20
Components of Rate Decrease - Other Expenses Subject to Cost Recovery Mechanisms | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		80
Components of Rate Decrease - Depreciation and Amortization | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		115
Components of Rate Decrease - Higher Depreciation Rates | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		35
Components of Rate Decrease - PISA Eligible Depreciation | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		35
Components of Rate Decrease - Return on Rate Base | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		60
Components of Rate Decrease - PISA Eligible Return on Rate Base | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 30
Wind Generation Facility | Union Electric Company
Rate And Regulatory Matters [Line Items]
Amount of Megawatts | MWh		300		157	400
Estimated Capital Project Costs						$ 1,200
MEEIA 2013 & 2016 | Final Rate Order | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Total operating revenues			$ 20
MEEIA 2016 | Final Rate Order | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Total operating revenues		$ 18	$ 5
Subsequent Event | Pending Rate Case | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Number of Months Between Callaway Maintenance Outages	18 months
Minimum | Pending Rate Case | Natural Gas | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage		9.40%
Maximum | Pending Rate Case | Natural Gas | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage		9.95%