Amount of megawatts

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The percentage of depreciation not allowed to be deferred under the PISA election.

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Estimated Capital Project Costs

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Frequency Rates Must be Reset to Use the FAC

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Infrastructure System Replacement Surcharge Return on Equity Percentage

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Number of months to complete MoPSC electric service proceeding

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Amount of public utility's requested rate increase (decrease) over interim rates with regulatory agency.

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Rate And Regulatory Matters [Line Items]

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Value established for a utility on which the entity is allowed to earn a particular rate of return

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Percentage of public utility's requested equity to capital, including debt and equity.

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Amount of public utility's requested rate increase (decrease) with regulatory agency.

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Percentage of public utility's requested return on equity.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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Rate And Regulatory Matters (Narrative-Missouri) (Detail) $ in Millions	3 Months Ended				6 Months Ended
	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($) MWh	Dec. 31, 2018 MWh	Jun. 30, 2018 USD ($) MWh	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)
Rate And Regulatory Matters [Line Items]
Total operating revenues		$ 1,379		$ 1,563	$ 2,935	$ 3,148
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%
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 [Domain] \| 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						$ 5
Subsequent Event \| Pending Rate Case [Member] \| 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
Subsequent Event \| Pending Rate Case [Member] \| 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
Subsequent Event \| Components of Rate Decrease - Net Energy Costs [Member] \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	100
Subsequent Event \| Components of Rate Decrease - Customer Sales Volume [Member] \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	55
Subsequent Event \| Components of Rate Decrease - Other Expenses [Member] \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	20
Subsequent Event \| Components of Rate Decrease - Other Expenses Subject to Cost Recovery Mechanisms [Member] \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	80
Subsequent Event \| Components of Rate Decrease - Depreciation and Amortization [Member] \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	115
Subsequent Event \| Components of Rate Decrease - Higher Depreciation Rates [Member] \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	35
Subsequent Event \| Components of Rate Decrease - PISA Eligible Depreciation [Member] \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	35
Subsequent Event \| Components of Rate Decrease - Return on Rate Base [Member] \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	60
Subsequent Event \| Components of Rate Decrease - PISA Eligible Return on Rate Base [Member] \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 30
Minimum \| Subsequent Event \| Pending Rate Case [Member] \| Natural Gas \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage	9.40%
Maximum \| Subsequent Event \| Pending Rate Case [Member] \| Natural Gas \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage	9.95%