Long-Term Debt And Equity Financings (Schedule Of Long-Term Debt Outstanding) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Nov. 26, 2019	Oct. 01, 2019	Sep. 30, 2019	Apr. 02, 2018
Debt Instrument [Line Items]
Debt instrument face amount	$ 1,000,000
Less: Maturities due within one year	(442,000,000)	$ (580,000,000)
Long-term Debt, Net	8,915,000,000	7,859,000,000
2022	505,000,000
2024	850,000,000
Thereafter	7,397,000,000
Ameren (parent)
Debt Instrument [Line Items]
Long-term debt, gross	1,150,000,000	700,000,000
Debt Issuance Costs, Net	(6,000,000)	(3,000,000)
Long-term Debt, Net	794,000,000	697,000,000
2024	450,000,000
Thereafter	350,000,000
Union Electric Company
Debt Instrument [Line Items]
Long-term debt, gross	4,229,000,000	4,029,000,000
Debt Issuance Costs, Net	(30,000,000)	(22,000,000)
Less: Maturities due within one year	(92,000,000)	(580,000,000)
Long-term Debt, Net	4,098,000,000	3,418,000,000
Less: Unamortized discount and premium	(9,000,000)	(9,000,000)
Debt Securities, Held-to-maturity	263,000,000	$ 270,000,000
2022	55,000,000
2024	350,000,000
Thereafter	$ 3,484,000,000
Union Electric Company | 1992 Series due 2022
Debt Instrument [Line Items]
Debt Instrument, Interest Rate During Period	2.58%	2.37%
Union Electric Company | 1998 Series A due 2033
Debt Instrument [Line Items]
Debt Instrument, Interest Rate During Period	3.43%	2.76%
Union Electric Company | 1998 Series B due 2033
Debt Instrument [Line Items]
Debt Instrument, Interest Rate During Period	3.57%	2.79%
Union Electric Company | 1998 Series C due 2033
Debt Instrument [Line Items]
Debt Instrument, Interest Rate During Period	3.43%	2.83%
Union Electric Company | City Of Bowling Green Agreement - Peno Creek Ct
Debt Instrument [Line Items]
Debt instrument face amount	$ 23,000,000	$ 30,000,000
Union Electric Company | Audrain County Agreement - Audrain County Ct
Debt Instrument [Line Items]
Debt instrument face amount	240,000,000	240,000,000
Ameren Illinois Company
Debt Instrument [Line Items]
Long-term debt, gross	3,613,000,000	3,330,000,000
Debt Issuance Costs, Net	(34,000,000)	(31,000,000)
Long-term Debt, Net	3,575,000,000	3,296,000,000
Less: Unamortized discount and premium	(4,000,000)	$ (3,000,000)
2022	400,000,000
Thereafter	$ 3,213,000,000
Ameren Illinois Company | Series B-1 1993 Due 2028
Debt Instrument [Line Items]
Debt Instrument, Interest Rate During Period	1.68%	1.58%
Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Long-term debt, gross	$ 450,000,000	$ 450,000,000
Debt Issuance Costs, Net	(2,000,000)	(2,000,000)
Long-term Debt, Net	448,000,000	448,000,000
2022	50,000,000
2024	50,000,000
Thereafter	350,000,000
Unsecured Debt | Senior Unsecured Notes 2.50% Due 2024 [Domain]
Debt Instrument [Line Items]
Debt instrument face amount	$ 450,000,000
Long-term debt interest rate	2.50%
Unsecured Debt | Ameren (parent) | Senior Unsecured Notes270 due 2020
Debt Instrument [Line Items]
Debt instrument face amount	$ 350,000,000	350,000,000
Long-term debt interest rate	2.70%
Unsecured Debt | Ameren (parent) | Senior Unsecured Notes 2.50% Due 2024 [Domain]
Debt Instrument [Line Items]
Debt instrument face amount	$ 450,000,000	0
Long-term debt interest rate	2.50%
Unsecured Debt | Ameren (parent) | Senior Unsecured Notes365 due 2026
Debt Instrument [Line Items]
Debt instrument face amount	$ 350,000,000	350,000,000
Long-term debt interest rate	3.65%
Unsecured Debt | Ameren Transmission Company of Illinois | Senior Unsecured Notes, 3.43%, Due 2050
Debt Instrument [Line Items]
Debt instrument face amount	$ 450,000,000	450,000,000
Secured Debt | Union Electric Company | 6.70% Senior secured notes due 2019
Debt Instrument [Line Items]
Debt instrument face amount	$ 0	329,000,000
Long-term debt interest rate	6.70%
Secured Debt | Union Electric Company | 5.10% Senior secured notes due 2019
Debt Instrument [Line Items]
Debt instrument face amount	$ 0	244,000,000
Long-term debt interest rate	5.10%
Secured Debt | Union Electric Company | 5.00% Senior secured notes due 2020
Debt Instrument [Line Items]
Debt instrument face amount	$ 85,000,000	85,000,000
Long-term debt interest rate	5.00%
Secured Debt | Union Electric Company | Senior Secured Notes350 Due2024
Debt Instrument [Line Items]
Debt instrument face amount	$ 350,000,000	350,000,000
Long-term debt interest rate	3.50%
Secured Debt | Union Electric Company | Senior Secured Notes, 2.95%, Due 2027
Debt Instrument [Line Items]
Debt instrument face amount	$ 400,000,000	400,000,000
Long-term debt interest rate	2.95%
Secured Debt | Union Electric Company | First Mortgage Bonds, 5.45%, Due 2028 [Domain]
Debt Instrument [Line Items]
Debt instrument face amount	$ 0
Long-term debt interest rate	5.45%
Secured Debt | Union Electric Company | First Mortgage Bonds, 3.50%, Due 2029 - $450 Issuance [Member]
Debt Instrument [Line Items]
Debt instrument face amount	$ 450,000,000	0
Long-term debt interest rate	3.50%
Secured Debt | Union Electric Company | 5.50% Senior secured notes due 2034
Debt Instrument [Line Items]
Debt instrument face amount	$ 184,000,000	184,000,000
Long-term debt interest rate	5.50%
Secured Debt | Union Electric Company | 5.30% Senior secured notes due 2037
Debt Instrument [Line Items]
Debt instrument face amount	$ 300,000,000	300,000,000
Long-term debt interest rate	5.30%
Secured Debt | Union Electric Company | 8.45% Senior secured notes due 2039
Debt Instrument [Line Items]
Debt instrument face amount	$ 350,000,000	350,000,000
Long-term debt interest rate	8.45%
Secured Debt | Union Electric Company | 3.90% Senior secured notes due 2042
Debt Instrument [Line Items]
Debt instrument face amount	$ 485,000,000	485,000,000
Long-term debt interest rate	3.90%
Secured Debt | Union Electric Company | Senior Secured Notes, 3.65%, Due 2045
Debt Instrument [Line Items]
Debt instrument face amount	$ 400,000,000	400,000,000
Long-term debt interest rate	3.65%
Secured Debt | Union Electric Company | First Mortgage Bonds, 4.00%, Due 2048 - $425 Issuance [Member]
Debt Instrument [Line Items]
Debt instrument face amount	$ 425,000,000	425,000,000				$ 425,000,000
Long-term debt interest rate	4.00%					4.00%
Secured Debt | Union Electric Company | First Mortgage Bonds, 3.25%, Due 2049 - $330 Issuance [Member]
Debt Instrument [Line Items]
Debt instrument face amount	$ 330,000,000	0		$ 330,000,000
Long-term debt interest rate	3.25%			3.25%
Secured Debt | Ameren Illinois Company | Senior Secured Notes, 2.70%, Due 2022
Debt Instrument [Line Items]
Debt instrument face amount	$ 400,000,000	400,000,000
Long-term debt interest rate	2.70%
Secured Debt | Ameren Illinois Company | Senior Secured Notes, 3.25%, Due 2025
Debt Instrument [Line Items]
Debt instrument face amount	$ 300,000,000	300,000,000
Long-term debt interest rate	3.25%
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.125% Due 2028
Debt Instrument [Line Items]
Debt instrument face amount	$ 60,000,000	60,000,000
Long-term debt interest rate	6.125%
Secured Debt | Ameren Illinois Company | First Mortgage Bonds, 3.80%, Due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument face amount	$ 430,000,000	$ 430,000,000
Long-term debt interest rate	3.80%	3.80%
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.70% Due 2036
Debt Instrument [Line Items]
Debt instrument face amount	$ 61,000,000	$ 61,000,000
Long-term debt interest rate	6.70%
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.70% Due 2036
Debt Instrument [Line Items]
Debt instrument face amount	$ 42,000,000	42,000,000
Long-term debt interest rate	6.70%
Secured Debt | Ameren Illinois Company | Senior Secured Notes 4.80% Due 2043
Debt Instrument [Line Items]
Debt instrument face amount	$ 280,000,000	280,000,000
Long-term debt interest rate	4.80%
Secured Debt | Ameren Illinois Company | Senior Secured Notes 4.30% Due 2044
Debt Instrument [Line Items]
Debt instrument face amount	$ 250,000,000	250,000,000
Long-term debt interest rate	4.30%
Secured Debt | Ameren Illinois Company | Senior Secured Notes, 4.15%, Due 2046
Debt Instrument [Line Items]
Debt instrument face amount	$ 490,000,000	490,000,000
Long-term debt interest rate	4.15%
Secured Debt | Ameren Illinois Company | Senior Secured Notes, 3.70%, Due 2047
Debt Instrument [Line Items]
Debt instrument face amount	$ 500,000,000	500,000,000
Long-term debt interest rate	3.70%
Secured Debt | Ameren Illinois Company | First Mortgage Bonds, 4.50%, Due 2049 [Member]
Debt Instrument [Line Items]
Debt instrument face amount	$ 500,000,000	$ 500,000,000
Long-term debt interest rate	4.50%	4.50%
Secured Debt | Ameren Illinois Company | First Mortgage Bonds, 3.25%, Due 2050 - $300 [Member]
Debt Instrument [Line Items]
Debt instrument face amount	$ 300,000,000	$ 0	$ 300,000,000
Long-term debt interest rate	3.25%		3.25%
Secured Debt | Ameren Illinois Company | First Mortgage Bonds, 5.90%, Due 2023 - Less Than $1 Million Outstanding [Domain]
Debt Instrument [Line Items]
Debt instrument face amount	$ 0	1,000,000			$ 1,000,000
Long-term debt interest rate	5.90%
Secured Debt | Ameren Illinois Company | First Mortgage Bonds, 5.70%, Due 2024 - Less Than $1 Million Outstanding [Domain]
Debt Instrument [Line Items]
Debt instrument face amount	$ 0	1,000,000			$ 1,000,000
Long-term debt interest rate	5.70%
Secured Debt | Ameren Transmission Company of Illinois | Senior Unsecured Notes, 3.43%, Due 2050
Debt Instrument [Line Items]
Long-term debt interest rate	3.43%
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1992 Series due 2022
Debt Instrument [Line Items]
Debt instrument face amount	$ 47,000,000	47,000,000
Long-term debt interest rate	1.60%
Debt Instrument, Interest Rate During Period	2.58%
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series A due 2033
Debt Instrument [Line Items]
Debt instrument face amount	$ 60,000,000	60,000,000
Long-term debt interest rate	2.90%
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series B due 2033
Debt Instrument [Line Items]
Debt instrument face amount	$ 50,000,000	50,000,000
Long-term debt interest rate	2.90%
Debt Instrument, Interest Rate During Period	3.57%
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series C due 2033
Debt Instrument [Line Items]
Debt instrument face amount	$ 50,000,000	50,000,000
Long-term debt interest rate	2.75%
Debt Instrument, Interest Rate During Period	3.43%
Environmental Improvement And Pollution Control Revenue Bonds | Ameren Illinois Company | Series 1993 5.90% Due 2023
Debt Instrument [Line Items]
Debt instrument face amount	$ 0
Environmental Improvement And Pollution Control Revenue Bonds | Ameren Illinois Company | Series 1994 A 5.70% Due 2024
Debt Instrument [Line Items]
Debt instrument face amount	0
Environmental Improvement And Pollution Control Revenue Bonds | Ameren Illinois Company | Series B-1 1993 Due 2028
Debt Instrument [Line Items]
Debt instrument face amount	$ 0	$ 17,000,000
Redemption price, percentage	100.00%
Senior Notes [Member] | Ameren Transmission | Senior Unsecured Notes, 3.43%, Due 2050
Debt Instrument [Line Items]
Long-term debt interest rate	3.43%
Maximum | Unsecured Debt | Ameren Transmission Company of Illinois | Senior Unsecured Notes, 3.43%, Due 2050
Debt Instrument [Line Items]
Debt instrument face amount	$ 450,000,000.0
Debt Instrument, Redemption, Period One | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
2022	49,500,000
Debt Instrument, Redemption, Period Two | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
2024	49,500,000
Debt Instrument, Redemption, Period Three | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Thereafter	49,500,000
Debt Instrument, Redemption, Period Four | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Thereafter	49,500,000
Debt Instrument, Redemption, Period Five | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Thereafter	49,500,000
Debt Instrument, Redemption, Period Six | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Thereafter	49,500,000
Debt Instrument, Redemption, Period Seven | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Thereafter	76,500,000
Debt Instrument, Redemption, Period Eight | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Thereafter	$ 76,500,000