Achieved Percentage of Energy Efficiency Earnings For Incentive Award

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- Definition

Amount of megawatts

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- Definition

Annual Incentive Award if Energy Efficiency Goals Are Achieved

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- Definition

Estimated incremental capital investment to modernize the electric grid

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- Definition

Incentive Award if Energy Efficiency Goals Are Achieved

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- Definition

Ameren Missouri's electric energy efficiency investments under the MEEIA.

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- Definition

Deferral percentage of the depreciation expense on certain property, plant, and equipment placed in-service and not included in base rates.

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- Definition

Percentage of energy customers receive from renewable resources

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- Definition

Rate And Regulatory Matters [Line Items]

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- Definition

Recovery period for plant in service accounting deferrals

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- Definition

Amount of public utility's approved rate increase (decrease) by regulatory agency.

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Percentage of public utility's approved rate increase (decrease) by regulatory agency.

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- Definition

The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.

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- Definition

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
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) MWh	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Rate And Regulatory Matters [Line Items]
Regulatory Liability, Noncurrent	$ 4,540		$ 4,540		$ 4,387
Total operating revenues	1,563	$ 1,537	3,148	$ 3,052
Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory Liability, Noncurrent	$ 2,754		$ 2,754		$ 2,664
PISA Deferral Percentage			85.00%
Recovery period for plant in service accounting deferrals			20 years
MEEIAEnergyEfficiencyInvestments			$ 92
Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount			167
Estimated incremental capital Investment			1,000
Incentive Award if Energy Efficiency Goals Are Achieved			$ 115
Achieved Percentage of Energy Efficiency Earnings For Incentive Award	100.00%		100.00%
TCJA - Excess Amounts Collected in Rates
Rate And Regulatory Matters [Line Items]
Regulatory Liability, Noncurrent	$ 47		$ 47
TCJA - Excess Accumulated Deferred Income Tax Amortization \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount			$ 74
Wind Generation Facility \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Amount of Megawatts \| MWh			400
Renewable Choice Program \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Amount of Megawatts \| MWh			400
Renewable Choice Program \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Percentage of Energy Received From Renewable Resources	100.00%
Renewable Choice Program - Megawatts Owned [Member] \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Amount of Megawatts \| MWh			200
MEEIA 2016 \| Final Rate Order \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Total operating revenues			$ 5
MEEIA 2016 \| Final Rate Order \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved			$ 27
Achieved Percentage of Energy Efficiency Earnings For Incentive Award	100.00%		100.00%
MEEIA 2013 \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved			$ 9
Minimum \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Annual Incentive Award if Energy Efficiency Goals Are Achieved			$ 10
Maximum
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage			2.85%
Maximum \| Electricity \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Annual Incentive Award if Energy Efficiency Goals Are Achieved			$ 24