Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	3 Months Ended
Mar. 31, 2020 USD ($) MWh
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	$ 569
2021	499
2022	305
2023	218
2024	148
Thereafter	197
Total unrecorded unconditional purchase obligation	1,936
Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	242
2021	219
2022	185
2023	105
2024	94
Thereafter	55
Total unrecorded unconditional purchase obligation	900
Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	150
2021	129
2022	69
2023	39
2024	13
Thereafter	43
Total unrecorded unconditional purchase obligation	443
Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	35
2021	57
2022	11
2023	44
2024	15
Thereafter	16
Total unrecorded unconditional purchase obligation	178
Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	85
2021	51
2022	13
2023	3
2024	0
Thereafter	0
Total unrecorded unconditional purchase obligation	$ 152
Amount of Megawatts \| MWh	102
Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	$ 2
2021	3
2022	3
2023	3
2024	3
Thereafter	24
Total unrecorded unconditional purchase obligation	38
Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	55
2021	40
2022	24
2023	24
2024	23
Thereafter	59
Total unrecorded unconditional purchase obligation	225
Renewable Energy Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	15
Zero Emission Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	11
Union Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	354
2021	336
2022	237
2023	189
2024	141
Thereafter	140
Total unrecorded unconditional purchase obligation	1,397
Union Electric Company \| Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	242
2021	219
2022	185
2023	105
2024	94
Thereafter	55
Total unrecorded unconditional purchase obligation	900
Union Electric Company \| Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	32
2021	26
2022	15
2023	13
2024	6
Thereafter	19
Total unrecorded unconditional purchase obligation	111
Union Electric Company \| Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	35
2021	57
2022	11
2023	44
2024	15
Thereafter	16
Total unrecorded unconditional purchase obligation	178
Union Electric Company \| Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	0
2021	0
2022	0
2023	0
2024	0
Thereafter	0
Total unrecorded unconditional purchase obligation	$ 0
Amount of Megawatts \| MWh	102
Union Electric Company \| Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	$ 2
2021	3
2022	3
2023	3
2024	3
Thereafter	24
Total unrecorded unconditional purchase obligation	38
Union Electric Company \| Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	43
2021	31
2022	23
2023	24
2024	23
Thereafter	26
Total unrecorded unconditional purchase obligation	170
Ameren Illinois Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	208
2021	158
2022	67
2023	29
2024	7
Thereafter	24
Total unrecorded unconditional purchase obligation	493
Ameren Illinois Company \| Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	0
2021	0
2022	0
2023	0
2024	0
Thereafter	0
Total unrecorded unconditional purchase obligation	0
Ameren Illinois Company \| Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	118
2021	103
2022	54
2023	26
2024	7
Thereafter	24
Total unrecorded unconditional purchase obligation	332
Ameren Illinois Company \| Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	0
2021	0
2022	0
2023	0
2024	0
Thereafter	0
Total unrecorded unconditional purchase obligation	0
Ameren Illinois Company \| Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	85
2021	51
2022	13
2023	3
2024	0
Thereafter	0
Total unrecorded unconditional purchase obligation	152
Ameren Illinois Company \| Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	0
2021	0
2022	0
2023	0
2024	0
Thereafter	0
Total unrecorded unconditional purchase obligation	0
Ameren Illinois Company \| Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	5
2021	4
2022	0
2023	0
2024	0
Thereafter	0
Total unrecorded unconditional purchase obligation	9
Ameren Illinois Company \| Renewable Energy Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	15
Ameren Illinois Company \| Zero Emission Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Total unrecorded unconditional purchase obligation	$ 11
Ameren Illinois Company \| April 2020 Procurement
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Amount of Megawatthours \| MWh	3,550,800
Long-term Purchase Commitment, Amount	$ 92
Ameren Illinois Company \| April 2020 Procurement - Capacity
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Amount of Megawatts \| MWh	617
Long-term Purchase Commitment, Amount	$ 4

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Amount of megawatts

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- Definition

The minimum amount the entity agreed to spend under the long-term purchase commitment.

+ References

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 440
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77896938&loc=d3e25336-109308

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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- Definition

Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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- Definition

Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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- Definition

Amount of the unrecorded obligation to transfer funds in the future for fixed or minimum amounts or quantities of goods or services at fixed or minimum prices (for example, as in take-or-pay contracts or throughput contracts).

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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