Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount of investment in debt security measured at amortized cost (held-to-maturity).

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Long-Term Debt And Equity Financings (Schedule Of Long-Term Debt Outstanding) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Oct. 30, 2020	Feb. 03, 2020	Dec. 31, 2019	Nov. 26, 2019	Oct. 01, 2019
Debt Instrument [Line Items]
Less: Maturities due within one year	$ (8,000,000)			$ (442,000,000)
Long-term Debt, Net	11,078,000,000			8,915,000,000
August 2022	505,000,000
August 2024	850,000,000
Thereafter	9,287,000,000
Ameren (parent)
Debt Instrument [Line Items]
Long-term debt, gross	1,600,000,000			1,150,000,000
Less: Unamortized discount and premium	(2,000,000)			0
Debt Issuance Costs, Net	(10,000,000)			(6,000,000)
Long-term Debt, Net	1,588,000,000			794,000,000
August 2022	0
August 2024	450,000,000
Thereafter	1,150,000,000
Union Electric Company
Debt Instrument [Line Items]
Long-term debt, gross	5,152,000,000			4,229,000,000
Less: Unamortized discount and premium	(12,000,000)			(9,000,000)
Debt Issuance Costs, Net	(36,000,000)			(30,000,000)
Less: Maturities due within one year	(8,000,000)			(92,000,000)
Long-term Debt, Net	5,096,000,000			4,098,000,000
Debt Securities, Held-to-maturity	256,000,000			263,000,000
August 2022	55,000,000
August 2024	350,000,000
Thereafter	4,499,000,000
Union Electric Company \| City Of Bowling Green Agreement - Peno Creek Ct
Debt Instrument [Line Items]
Debt instrument face amount	16,000,000			23,000,000
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	3,988,000,000			3,613,000,000
Less: Unamortized discount and premium	(6,000,000)			(4,000,000)
Debt Issuance Costs, Net	(36,000,000)			(34,000,000)
Long-term Debt, Net	3,946,000,000			3,575,000,000
August 2022	400,000,000
August 2024	0
Thereafter	3,288,000,000
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
August 2022	50,000,000
August 2024	50,000,000
Thereafter	350,000,000
Unsecured Debt \| Senior Unsecured Notes 2.50% Due 2024
Debt Instrument [Line Items]
Debt instrument face amount	450,000,000
Unsecured Debt \| Ameren (parent) \| Senior Unsecured Notes 2.70% due 2020
Debt Instrument [Line Items]
Debt instrument face amount	$ 0	$ 350,000,000		350,000,000
Long-term debt interest rate	2.70%
Unsecured Debt \| Ameren (parent) \| Senior Unsecured Notes 2.50% Due 2024
Debt Instrument [Line Items]
Debt instrument face amount	$ 450,000,000			450,000,000
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 (parent) \| Senior Secured Notes 3.50% Due 2031
Debt Instrument [Line Items]
Debt instrument face amount	$ 800,000,000			0
Long-term debt interest rate	3.50%
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			450,000,000
Secured Debt \| 5.00% Senior secured notes due 2020
Debt Instrument [Line Items]
Debt instrument face amount			$ 85,000,000
Secured Debt \| Union Electric Company \| 5.00% Senior secured notes due 2020
Debt Instrument [Line Items]
Debt instrument face amount	$ 0			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, 3.50%, Due 2029 - $450 Issuance
Debt Instrument [Line Items]
Debt instrument face amount	$ 450,000,000			450,000,000
Long-term debt interest rate	3.50%
Secured Debt \| Union Electric Company \| First Mortgage Bonds, 2.95%, Due 2030 - $465 Issuance
Debt Instrument [Line Items]
Debt instrument face amount	$ 465,000,000			0
Long-term debt interest rate	2.95%
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
Debt Instrument [Line Items]
Debt instrument face amount	$ 425,000,000			425,000,000
Long-term debt interest rate	4.00%
Secured Debt \| Union Electric Company \| First Mortgage Bonds, 3.25%, Due 2049 - $330 Issuance
Debt Instrument [Line Items]
Debt instrument face amount	$ 330,000,000			330,000,000		$ 330,000,000
Long-term debt interest rate	3.25%
Secured Debt \| Union Electric Company \| First Mortgage Bonds, 2.625%, Due 2051 - $550 Issuance
Debt Instrument [Line Items]
Debt instrument face amount	$ 550,000,000			0
Long-term debt interest rate	2.625%
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
Debt Instrument [Line Items]
Debt instrument face amount	$ 430,000,000			430,000,000
Long-term debt interest rate	3.80%
Secured Debt \| Ameren Illinois Company \| First Mortgage Bonds, 1.55%, Due 2030
Debt Instrument [Line Items]
Debt instrument face amount	$ 375,000,000			0
Long-term debt interest rate	1.55%
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
Debt Instrument [Line Items]
Debt instrument face amount	$ 500,000,000			500,000,000
Long-term debt interest rate	4.50%
Secured Debt \| Ameren Illinois Company \| First Mortgage Bonds, 3.25%, Due 2050 - $300
Debt Instrument [Line Items]
Debt instrument face amount	$ 300,000,000			300,000,000	$ 300,000,000
Long-term debt interest rate	3.25%
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%
Senior Notes \| Ameren Transmission \| Senior Unsecured Notes, 3.43%, Due 2050
Debt Instrument [Line Items]
Long-term debt interest rate	3.43%
Debt Instrument, Redemption, Period One \| Unsecured Debt \| Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
August 2022	$ 49,500,000
Debt Instrument, Redemption, Period Two \| Unsecured Debt \| Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
August 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