Long-Term Debt And Equity Financings (Schedule Of Long-Term Debt Outstanding) (Details) - USD ($)		12 Months Ended
		Dec. 31, 2016		Mar. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Less: Maturities due within one year		$ (681,000,000)			$ (395,000,000)
Long-term Debt, Net		6,595,000,000			6,880,000,000
Parent Company
Debt Instrument [Line Items]
Long-term debt, gross		700,000,000			700,000,000
Debt Issuance Costs, Net		(6,000,000)			(6,000,000)
Long-term Debt, Net		$ 694,000,000			694,000,000
Parent Company | Senior Unsecured Notes270 due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt interest rate		2.70%
Long-term debt maturity date		2020
Parent Company | Senior Unsecured Notes365 due 2026 [Member]
Debt Instrument [Line Items]
Long-term debt interest rate		3.65%
Long-term debt maturity date		2026
Ameren Illinois Company
Debt Instrument [Line Items]
Long-term debt, gross		$ 2,607,000,000			2,496,000,000
Less: Unamortized discount and premium		0			(7,000,000)
Debt Issuance Costs, Net		(19,000,000)			(18,000,000)
Less: Maturities due within one year		(250,000,000)			(129,000,000)
Long-term Debt, Net		2,338,000,000			2,342,000,000
Union Electric Company
Debt Instrument [Line Items]
Long-term debt, gross		4,019,000,000			4,135,000,000
Less: Unamortized discount and premium		(6,000,000)			(6,000,000)
Debt Issuance Costs, Net		(19,000,000)			(19,000,000)
Less: Maturities due within one year		(431,000,000)			(266,000,000)
Long-term Debt, Net		3,563,000,000			3,844,000,000
Union Electric Company | City Of Bowling Green Capital Lease Peno Creek Ct
Debt Instrument [Line Items]
Capital lease obligations		$ 42,000,000			48,000,000
Long-term debt maturity date		2022
Union Electric Company | Audrain County Capital Lease Audrain County Ct
Debt Instrument [Line Items]
Capital lease obligations		$ 240,000,000			240,000,000
Long-term debt maturity date		2023
Unsecured Debt [Member] | Parent Company | Senior Unsecured Notes270 due 2020 [Member]
Debt Instrument [Line Items]
Debt instrument face amount		$ 350,000,000			350,000,000
Long-term debt interest rate		2.70%
Unsecured Debt [Member] | Parent Company | Senior Unsecured Notes365 due 2026 [Member]
Debt Instrument [Line Items]
Debt instrument face amount		$ 350,000,000			350,000,000
Long-term debt interest rate		3.65%
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.20% Due 2016
Debt Instrument [Line Items]
Debt instrument face amount		$ 0			54,000,000
Long-term debt interest rate		6.20%
Long-term debt maturity date		2016
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.25% Due 2016
Debt Instrument [Line Items]
Debt instrument face amount		$ 0			75,000,000
Long-term debt interest rate		6.25%
Long-term debt maturity date		2016
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.125% Due 2017
Debt Instrument [Line Items]
Debt instrument face amount	[1]	$ 250,000,000	[2]		250,000,000	[3]
Long-term debt interest rate		6.125%
Long-term debt maturity date		2017
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.25% Due 2018
Debt Instrument [Line Items]
Debt instrument face amount	[1],[2]	$ 144,000,000			144,000,000
Long-term debt interest rate		6.25%
Long-term debt maturity date		2018
Secured Debt | Ameren Illinois Company | Senior Secured Notes 9.75% Due 2018
Debt Instrument [Line Items]
Debt instrument face amount	[1],[2]	$ 313,000,000			313,000,000
Long-term debt interest rate		9.75%
Long-term debt maturity date		2018
Secured Debt | Ameren Illinois Company | Senior Secured Notes, 2.70%, Due 2022
Debt Instrument [Line Items]
Debt instrument face amount	[1],[2]	$ 400,000,000			400,000,000
Long-term debt interest rate		2.70%
Long-term debt maturity date		2022
Secured Debt | Ameren Illinois Company | Senior Secured Notes, 3.25%, Due 2025
Debt Instrument [Line Items]
Debt instrument face amount	[2]	$ 300,000,000			300,000,000
Long-term debt interest rate		3.25%
Long-term debt maturity date		2025
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.125% Due 2028
Debt Instrument [Line Items]
Debt instrument face amount	[2]	$ 60,000,000			60,000,000
Long-term debt interest rate		6.125%
Long-term debt maturity date		2028
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.70% Due 2036
Debt Instrument [Line Items]
Debt instrument face amount	[2]	$ 61,000,000			61,000,000
Long-term debt interest rate		6.70%
Long-term debt maturity date		2036
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.70% Due 2036
Debt Instrument [Line Items]
Debt instrument face amount	[4]	$ 42,000,000			42,000,000
Long-term debt interest rate		6.70%
Long-term debt maturity date		2036
Secured Debt | Ameren Illinois Company | Senior Secured Notes 4.80% Due 2043
Debt Instrument [Line Items]
Debt instrument face amount	[2]	$ 280,000,000			280,000,000
Long-term debt interest rate		4.80%
Long-term debt maturity date		2043
Secured Debt | Ameren Illinois Company | Senior Secured Notes 4.30% Due 2044
Debt Instrument [Line Items]
Debt instrument face amount	[2]	$ 250,000,000			250,000,000
Long-term debt interest rate		4.30%
Long-term debt maturity date		2044
Secured Debt | Ameren Illinois Company | Senior Secured Notes, 4.15%, Due 2046 [Member]
Debt Instrument [Line Items]
Debt instrument face amount	[2]	$ 490,000,000			250,000,000
Long-term debt interest rate		4.15%
Long-term debt maturity date		2046
Secured Debt | Ameren Illinois Company | Series 1993 5.90% Due 2023
Debt Instrument [Line Items]
Debt instrument face amount	[4]	$ 1,000,000
Long-term debt interest rate		5.90%
Secured Debt | Union Electric Company | 5.40% Senior secured notes due 2016
Debt Instrument [Line Items]
Debt instrument face amount		$ 0	[5]	$ 260,000,000	260,000,000	[5]
Long-term debt interest rate		5.40%
Long-term debt maturity date		2016
Secured Debt | Union Electric Company | 6.40% Senior secured notes due 2017
Debt Instrument [Line Items]
Debt instrument face amount	[5]	$ 425,000,000			425,000,000
Long-term debt interest rate		6.40%
Long-term debt maturity date		2017
Secured Debt | Union Electric Company | 6.00% Senior secured notes due 2018
Debt Instrument [Line Items]
Debt instrument face amount	[5],[6]	$ 179,000,000			179,000,000
Long-term debt interest rate		6.00%
Long-term debt maturity date		2018
Secured Debt | Union Electric Company | 5.10% Senior secured notes due 2018
Debt Instrument [Line Items]
Debt instrument face amount	[5]	$ 199,000,000			199,000,000
Long-term debt interest rate		5.10%
Long-term debt maturity date		2018
Secured Debt | Union Electric Company | 6.70% Senior secured notes due 2019
Debt Instrument [Line Items]
Debt instrument face amount	[5],[6]	$ 329,000,000			329,000,000
Long-term debt interest rate		6.70%
Long-term debt maturity date		2019
Secured Debt | Union Electric Company | 5.10% Senior secured notes due 2019
Debt Instrument [Line Items]
Debt instrument face amount	[5]	$ 244,000,000			244,000,000
Long-term debt interest rate		5.10%
Long-term debt maturity date		2019
Secured Debt | Union Electric Company | 5.00% Senior secured notes due 2020
Debt Instrument [Line Items]
Debt instrument face amount	[5]	$ 85,000,000			85,000,000
Long-term debt interest rate		5.00%
Long-term debt maturity date		2020
Secured Debt | Union Electric Company | Senior Secured Notes350 Due2024 [Member]
Debt Instrument [Line Items]
Debt instrument face amount	[5]	$ 350,000,000			350,000,000
Long-term debt interest rate		3.50%
Long-term debt maturity date		2024
Secured Debt | Union Electric Company | 5.50% Senior secured notes due 2034
Debt Instrument [Line Items]
Debt instrument face amount	[5]	$ 184,000,000			184,000,000
Long-term debt interest rate		5.50%
Long-term debt maturity date		2034
Secured Debt | Union Electric Company | 5.30% Senior secured notes due 2037
Debt Instrument [Line Items]
Debt instrument face amount	[5]	$ 300,000,000			300,000,000
Long-term debt interest rate		5.30%
Long-term debt maturity date		2037
Secured Debt | Union Electric Company | 8.45% Senior secured notes due 2039
Debt Instrument [Line Items]
Debt instrument face amount	[5],[6]	$ 350,000,000			350,000,000
Long-term debt interest rate		8.45%
Long-term debt maturity date		2039
Secured Debt | Union Electric Company | 3.90% Senior secured notes due 2042
Debt Instrument [Line Items]
Debt instrument face amount	[5],[6]	$ 485,000,000			485,000,000
Long-term debt interest rate		3.90%
Long-term debt maturity date		2042
Secured Debt | Union Electric Company | Senior Secured Notes, 3.65%, Due 2045 [Member]
Debt Instrument [Line Items]
Debt instrument face amount	[5]	$ 400,000,000			250,000,000
Long-term debt interest rate		3.65%
Long-term debt maturity date		2045
Environmental Improvement And Pollution Control Revenue Bonds | Ameren Illinois Company | Series 1993 5.90% Due 2023
Debt Instrument [Line Items]
Debt instrument face amount	[3]	$ 1,000,000	[4]		1,000,000
Long-term debt interest rate		5.90%
Long-term debt maturity date		2023
Redemption price, percentage		100.00%
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	[7]	$ 1,000,000	[4]		1,000,000
Long-term debt interest rate		5.70%
Long-term debt maturity date		2024
Redemption price, percentage		100.00%
Environmental Improvement And Pollution Control Revenue Bonds | Ameren Illinois Company | Series B-1 1993 Due 2028
Debt Instrument [Line Items]
Debt instrument face amount	[8],[9]	$ 17,000,000			17,000,000
Long-term debt maturity date		2028
Redemption price, percentage		100.00%
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1992 Series due 2022
Debt Instrument [Line Items]
Debt instrument face amount	[8],[10]	$ 47,000,000			47,000,000
Long-term debt maturity date		2022
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1993 5.45% Series due 2028
Debt Instrument [Line Items]
Debt instrument face amount	[11]	$ 1,000,000
Long-term debt interest rate		5.45%
Long-term debt maturity date		2028
Redemption price, percentage		100.00%
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series A due 2033
Debt Instrument [Line Items]
Debt instrument face amount	[8],[10]	$ 60,000,000			60,000,000
Long-term debt maturity date		2033
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series B due 2033
Debt Instrument [Line Items]
Debt instrument face amount	[8],[10]	$ 50,000,000			50,000,000
Long-term debt maturity date		2033
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series C due 2033
Debt Instrument [Line Items]
Debt instrument face amount	[8],[10]	$ 50,000,000			$ 50,000,000
Long-term debt maturity date		2033
Maximum | Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1992 Series due 2022
Debt Instrument [Line Items]
Long-term debt interest rate		18.00%
Maximum | Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series A due 2033
Debt Instrument [Line Items]
Long-term debt interest rate		0.00%
Maximum | Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series B due 2033
Debt Instrument [Line Items]
Long-term debt interest rate		0.00%
Maximum | Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series C due 2033
Debt Instrument [Line Items]
Long-term debt interest rate		0.00%

[1] (h)Ameren Illinois has agreed that so long as any of the 2.70% senior secured notes due 2022 are outstanding, Ameren Illinois will not permit a release date to occur, and so long as any of the 9.75% senior secured notes due 2018, 6.25% senior secured notes due 2018, and 6.125% senior secured notes due 2017 are outstanding, Ameren Illinois will not optionally redeem, purchase or otherwise retire in full the outstanding first mortgage bonds not subject to release provisions; therefore, a release date will not occur so long as any of these notes remain outstanding.

[2] These notes are collaterally secured by mortgage bonds issued by Ameren Illinois under its 1992 mortgage indenture. They are secured by substantially all property of the former IP and CIPS. The notes have a fall-away lien provision and will remain secured only as long as any series of first mortgage bonds issued under its 1992 mortgage indenture remain outstanding. Redemption, purchase, or maturity of all mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Illinois senior secured notes currently outstanding, we do not expect the mortgage bond lien protection associated with these notes to fall away before 2022.

[3] (i)These bonds are first mortgage bonds issued by Ameren Illinois under its 1933 mortgage indenture. They are secured by substantially all property of the former CILCO. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding.

[4] (f)These notes are collaterally secured by first mortgage bonds issued by Ameren Illinois under its 1933 mortgage indenture. The notes have a fall-away lien provision, and Ameren Illinois could cause these notes to become unsecured at any time by redeeming the pollution control bonds 5.90% Series 1993 due 2023 (of which less than $1 million remains outstanding).

[5] (a)These notes are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any first mortgage bonds issued under the Ameren Missouri mortgage indenture remain outstanding. Redemption, purchase, or maturity of all first mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the first mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Missouri senior secured notes currently outstanding, we do not expect the first mortgage bond lien protection associated with these notes to fall away before 2042.

[6] (b)Ameren Missouri has agreed that so long as any of the 3.90% senior secured notes due 2042 are outstanding, Ameren Missouri will not permit a release date to occur, and so long as any of the 6.70% senior secured notes due 2019, 6.00% senior secured notes due 2018, and 8.45% senior secured notes due 2039 are outstanding, Ameren Missouri will not optionally redeem, purchase, or otherwise retire in full the outstanding first mortgage bonds not subject to release provisions.

[7] (j)These bonds are mortgage bonds issued by Ameren Illinois under its 1992 mortgage indenture. They are secured by substantially all property of the former IP and CIPS. The bonds are callable at 100% of par value. The bonds are also backed by an insurance guarantee policy. Less than $1 million principal amount of the bonds remains outstanding.

[8] (d)The interest rates and the periods during which such rates apply vary depending on our selection of defined rate modes. Maximum interest rates could reach 18%, depending on the series of bonds. The bonds are callable at 100% of par value. The average interest rates for 2016 and 2015 were as follows: 2016 2015Ameren Missouri 1992 Series due 20220.66% 0.06%Ameren Missouri 1998 Series A due 20330.91% 0.24%Ameren Missouri 1998 Series B due 20330.92% 0.24%Ameren Missouri 1998 Series C due 20330.97% 0.24%Ameren Illinois 1993 Series B-1 due 20280.70% 0.49%

[9] (k)The bonds are callable at 100% of par value.

[10] c)These bonds are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture and have a fall-away lien provision similar to that of Ameren Missouri's senior secured notes. The bonds are also backed by an insurance guarantee policy.

[11] (e)These bonds are first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage bond indenture and are secured by substantially all Ameren Missouri property and franchises. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding.