Spread Applied To the Floating Interest Rate Factor Used to Adjust the Forward Sale Price

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The initial per share price of the Company's stock at which the contract holder of the freestanding contract has the right to purchase or sell the Company's stock at a future date.

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Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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The number of issuer's shares to which the forward contract is indexed.

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Current fair value of each cash settlement alternative for each freestanding forward contract.

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The cash inflow from the additional capital contribution to the entity.

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The cash inflow from amounts received from issuance of long-term debt that is wholly or partially secured by collateral. Excludes proceeds from tax exempt secured debt.

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The cash inflow from the issuance of long-term debt that is not secured by collateral. Excludes proceeds from tax exempt unsecured debt.

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Amount of cash outflow for the payment of debt classified as other, maturing after one year or the operating cycle, if longer.

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The fair value of stock issued in noncash financing activities.

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Number of shares of stock issued attributable to transactions classified as other.

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Long-Term debt and Equity Financings (Narrative) (Details) - USD ($) $ / shares in Units, shares in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2019	Oct. 30, 2019	Oct. 01, 2019	Aug. 05, 2019
Long-Term Debt And Equity Financings [Line Items]
Stock Issued During Period, Shares, New Issues	0.2	0.2	0.7	0.9
Issuance of common stock	$ 17,000,000		$ 54,000,000	$ 56,000,000
Stock Issued During Period, Shares, Other	0.0	0.0	0.8	0.7
Noncash financing activity – Issuance of common stock for stock-based compensation			$ 54,000,000	$ 35,000,000
Forward Contract Indexed to Issuer's Equity, Indexed Shares (in shares)								7.5
Forward Contract Indexed to Issuer's Equity, Initial Forward Rate (in dollars per share)								$ 74.18
Forward Contract Indexed To Issuer's Equity, Basis Spread on Variable Rate			0.75%
Forward Contract Indexed to Issuer's Equity, Settlement Alternatives, Cash, at Fair Value	$ 557,000,000		$ 557,000,000
Period End Net Cash Settlement Price	$ 47,000,000		$ 47,000,000
Period End Net Share Settlement Price (in shares)	0.6		0.6
Unsecured Debt [Member] \| First Mortgage Bonds, 2.50%, Due 2024 - $450 Issuance [Domain]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 450,000,000		$ 450,000,000
Debt Instrument, Interest Rate, Stated Percentage	2.50%		2.50%
Proceeds from Issuance of Debt			$ 447,000,000
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Noncash financing activity – Issuance of common stock for stock-based compensation			0	0
Union Electric Company \| Secured Debt \| First Mortgage Bonds, 3.50%, Due 2029 - $450 Issuance
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 450,000,000		$ 450,000,000
Debt Instrument, Interest Rate, Stated Percentage	3.50%		3.50%
Proceeds from Issuance of Secured Debt			$ 447,000,000
Union Electric Company \| Secured Debt \| Senior Secured Notes 6.70% Due 2019
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.70%		6.70%
Repayments of other long-term debt			$ 329,000,000
Union Electric Company \| Secured Debt \| 1992 Series Bonds, 1998 Series A Bonds, 1998 Series B Bonds, 1998 Series C Bonds [Domain]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 207,000,000		207,000,000
Union Electric Company \| Secured Debt \| First Mortgage Bonds, 5.45%, Due 2028 - Less Than $1 Million Outstanding [Domain]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 1,000,000		$ 1,000,000
Debt Instrument, Interest Rate, Stated Percentage	5.45%		5.45%
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Noncash financing activity – Issuance of common stock for stock-based compensation			$ 0	$ 0
Ameren Illinois Company \| Unsecured Debt [Member] \| Senior Unsecured 1993 Series B-1 [Domain]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 17,000,000		17,000,000
Ameren Illinois Company \| Secured Debt \| First Mortgage Bonds, 5.70%, Due 2024 - Less Than $1 Million Outstanding [Domain]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 1,000,000		$ 1,000,000
Debt Instrument, Interest Rate, Stated Percentage	5.70%		5.70%
Subsequent Event \| Union Electric Company \| Unsecured Debt [Member] \| Senior Unsecured Notes 5.10% Due 2019 [Domain]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Interest Rate, Stated Percentage							5.10%
Repayments of other long-term debt					$ 244,000,000
Subsequent Event \| Union Electric Company \| Secured Debt \| First Mortgage Bonds, 3.25%, Due 2049 - $330 Issuance [Member]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount							$ 330,000,000
Debt Instrument, Interest Rate, Stated Percentage					3.25%
Proceeds from Issuance of Secured Debt					$ 326,000,000
Subsequent Event \| Ameren Illinois Company \| Secured Debt \| First Mortgage Bonds, 5.90%, Due 2023 - Less Than $1 Million Outstanding [Domain]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount						$ 1,000,000
Debt Instrument, Interest Rate, Stated Percentage						5.90%
Subsequent Event \| Ameren Illinois Company \| Secured Debt \| Senior Secured Notes. 6.70%, Due 2023 [Domain]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Interest Rate, Stated Percentage						6.70%