Long-Term Debt And Equity Financings (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Feb. 03, 2020	Nov. 26, 2019	Oct. 01, 2019	Sep. 30, 2019	Aug. 05, 2019	Aug. 31, 2018	Apr. 02, 2018	Apr. 01, 2018
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized (in shares)	100,000,000
Preferred stock, par value (in dollars per share)	$ 0.01
Preferred stock, shares outstanding (in shares)	0
Stock Issued During Period, Shares, New Issues	900,000	1,200,000	0
Issuances of common stock	$ 68,000,000	$ 74,000,000	$ 0
Stock Issued During Period, Shares, Other	800,000	700,000	0
Stock Issued	$ 54,000,000	$ 35,000,000	$ 0
Common Stock, Shares Authorized Under 401(k) Plan	4,000,000
Common Stock, Shares Authorized Under DRPlus Plan	6,000,000
Forward Contract Indexed to Issuer's Equity, Indexed Shares								7,500,000
Forward Contract Indexed to Issuer's Equity, Initial Forward Rate								$ 74.18
Forward Contract Indexed to Issuer's Equity, Settlement Alternatives, Cash, at Fair Value	$ 555,000,000
Period End Net Cash Settlement Price	$ 25,000,000
Period End Net Share Settlement Price	300,000
Debt instrument face amount	$ 1,000,000
Repayments of Other Long-term Debt	580,000,000	841,000,000	681,000,000
Debt Default Provision Excess	$ 25,000,000
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized (in shares)	7,500,000
Preferred stock, par value (in dollars per share)	$ 1
Preferred stock, shares outstanding (in shares)	0
Stock Issued	$ 0	0	0
Repayments of Other Long-term Debt	$ 580,000,000	384,000,000	431,000,000
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized (in shares)	2,600,000
Preferred stock, par value (in dollars per share)	$ 0
Preferred stock, shares outstanding (in shares)	0
Stock Issued	$ 0	0	0
Repayments of Other Long-term Debt	$ 0	457,000,000	$ 250,000,000
Common stock equity to capitalization ratio	51.00%
Ameren Missouri and Ameren Illinois
Long-Term Debt And Equity Financings [Line Items]
Bonds interest rate assumption	5.00%
Dividend rate on preferred shares, percentage	6.00%
Senior Unsecured Notes 2.50% Due 2024 [Domain] | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 450,000,000
Long-term debt interest rate	2.50%
Proceeds from Issuance of Unsecured Debt	$ 447,000,000
5.00% Senior secured notes due 2020 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 85,000,000	85,000,000
Long-term debt interest rate	5.00%
First Mortgage Bonds, 3.50%, Due 2029 - $450 Issuance [Member] | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 450,000,000	0
Long-term debt interest rate	3.50%
Senior Secured Notes 6.70% due 2019 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 0	329,000,000
Long-term debt interest rate	6.70%
Repayments of Other Long-term Debt	$ 329,000,000
1992 Series Bonds, 1998 Series A Bonds, 1998 Series B Bonds, 1998 Series C Bonds [Domain] | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	207,000,000
First Mortgage Bonds, 3.25%, Due 2049 - $330 Issuance [Member] | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 330,000,000	0				$ 330,000,000
Long-term debt interest rate	3.25%					3.25%
Proceeds from issuance of secured debt	$ 326,000,000
Senior Unsecured Notes 5.10% Due 2019 [Domain] | Union Electric Company | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate						5.10%
Repayments of Other Long-term Debt	244,000,000
First Mortgage Bonds, 5.45%, Due 2028 - Less Than $1 Million Outstanding [Domain] | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount						$ 1,000,000
Long-term debt interest rate						5.45%
First Mortgage Bonds, 4.00%, Due 2048 - $425 Issuance [Member] | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 425,000,000	425,000,000								$ 425,000,000
Long-term debt interest rate	4.00%									4.00%
Proceeds from issuance of secured debt	$ 447,000,000	419,000,000
6.00% Senior secured notes due 2018 | Union Electric Company | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate											6.00%
Repayments of Other Long-term Debt		179,000,000
Senior Secured Notes 5.10% Due 2018 [Member] | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate									5.10%
Repayments of Other Long-term Debt		199,000,000
Senior Unsecured 1993 Series B-1 [Domain] | Ameren Illinois Company | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	17,000,000
First Mortgage Bonds, 5.70%, Due 2024 - Less Than $1 Million Outstanding [Domain] | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 0	1,000,000					$ 1,000,000
Long-term debt interest rate	5.70%
First Mortgage Bonds, 5.90%, Due 2023 - Less Than $1 Million Outstanding [Domain] | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 0	1,000,000					$ 1,000,000
Long-term debt interest rate	5.90%
Senior Secured Notes. 6.70%, Due 2036 [Domain] | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate	6.70%
First Mortgage Bonds, 3.25%, Due 2050 - $300 [Member] | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 300,000,000	0			$ 300,000,000
Long-term debt interest rate	3.25%				3.25%
Proceeds from issuance of secured debt	$ 296,000,000
First Mortgage Bonds, 3.80%, Due 2028 [Member] | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 430,000,000	$ 430,000,000
Long-term debt interest rate	3.80%	3.80%
Proceeds from issuance of secured debt		$ 427,000,000
Senior Secured Notes 6.25% Due 2018 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate											6.25%
Repayments of Other Long-term Debt		144,000,000
First Mortgage Bonds, 4.50%, Due 2049 [Member] | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 500,000,000	$ 500,000,000
Long-term debt interest rate	4.50%	4.50%
Proceeds from issuance of secured debt		$ 495,000,000
Senior Secured Notes 9.75% Due 2018 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate		9.75%
Repayments of Other Long-term Debt		$ 313,000,000
Senior Unsecured Notes, 3.43%, Due 2050 | Ameren Transmission Company of Illinois | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 450,000,000	$ 450,000,000
Ratio of Indebtedness to Net Capital	0.70
Ratio of Indebtedness to Total Assets	0.10
Senior Unsecured Notes, 3.43%, Due 2050 | Ameren Transmission Company of Illinois | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate	3.43%
Maximum | Senior Unsecured Notes, 3.43%, Due 2050 | Ameren Transmission Company of Illinois | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 450,000,000.0
Minimum | Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to capitalization ratio	30.00%
Subsequent Event | 5.00% Senior secured notes due 2020 | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount				$ 85,000,000
Long-term debt interest rate				5.00%