Long-Term Debt And Equity Financings (Narrative) (Details) (USD $)	12 Months Ended					1 Months Ended				3 Months Ended		1 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012	Jan. 31, 2014		May 31, 2014	Apr. 30, 2014	Dec. 31, 2013		Jun. 30, 2014	Dec. 31, 2014		Oct. 31, 2013	May 15, 2014
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized	100,000,000												100,000,000
Preferred stock, par value	$ 0.01												$ 0.01
Preferred stock, shares outstanding	0												0
Common stock, shares authorized	400,000,000		400,000,000							400,000,000			400,000,000
Repayments of Other Long-term Debt	$ 697,000,000		$ 399,000,000		$ 760,000,000
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized	7,500,000.0												7,500,000.0
Preferred stock, par value	$ 1												$ 1
Common stock, shares authorized	150,000,000		150,000,000							150,000,000			150,000,000
Repayments of Other Long-term Debt	109,000,000		249,000,000		427,000,000
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized	2,600,000.0												2,600,000.0
Common stock, shares authorized	45,000,000		45,000,000							45,000,000			45,000,000
Repayments of Other Long-term Debt	163,000,000		150,000,000		333,000,000
Debt instrument face amount						1,000,000	[1]
Common stock equity to capitalization ratio	53.00%												53.00%
Ameren Illinois Company | Environmental Improvement And Pollution Control Revenue Bonds
Long-Term Debt And Equity Financings [Line Items]
Redemptions of long-term debt						163,000,000
Parent Company
Long-Term Debt And Equity Financings [Line Items]
Common stock, shares authorized								8,600,000.0							4,000,000
Repayments of Other Long-term Debt	425,000,000		0		0
Ameren Missouri and Ameren Illinois
Long-Term Debt And Equity Financings [Line Items]
Bonds interest rate assumption	5.00%												5.00%
Dividend rate on preferred shares, percentage	6.00%
Series1993570 Due2024 [Member] | Ameren Illinois Company | Environmental Improvement And Pollution Control Revenue Bonds
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	1,000,000	[2]											1,000,000	[2]
Senior Unsecured Notes8875 Due2014 [Member] | Parent Company
Long-Term Debt And Equity Financings [Line Items]
Repayments of Other Long-term Debt								425,000,000
Debt instrument face amount			425,000,000							425,000,000
Long-term debt interest rate	8.875%							8.875%					8.875%
Senior Secured Notes, 3.50%, Due 2024 [Member] | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount									350,000,000
Long-term debt interest rate									3.50%
Proceeds from issuance of secured debt									348,000,000
5.50% Senior secured notes due 2014 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	0	[3]	104,000,000	[3]						104,000,000	[3]		0	[3]
Long-term debt interest rate	5.50%												5.50%			5.50%
Redemptions of long-term debt								104,000,000
Senior Secured Notes 4.80% Due 2043 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	280,000,000	[4]	280,000,000	[4]						280,000,000	[4]		280,000,000	[4]
Long-term debt interest rate	4.80%		4.80%							4.80%			4.80%
Proceeds from issuance of secured debt										276,000,000
Senior Secured Notes 8.875% Due 2013 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate			8.875%							8.875%
Redemptions of long-term debt										150,000,000
1993 5.45% Series due 2028 | Union Electric Company | Environmental Improvement And Pollution Control Revenue Bonds
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	1,000,000	[2]											1,000,000	[2]
Long-term debt interest rate	5.45%												5.45%		5.45%
Redemptions of long-term debt										44,000,000
4.65% Senior secured notes due 2013 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate															4.65%
Redemptions of long-term debt										200,000,000
Senior Secured Notes, 4.30%, Due 2044 [Member] [Member] | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount												250,000,000
Long-term debt interest rate												4.30%
Proceeds from issuance of secured debt												246,000,000
Senior Secured Notes, 3 Point 25, Due 2025 [Member] | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	300,000,000	[4],[5]											300,000,000	[4],[5]
Long-term debt interest rate	3.25%												3.25%
Proceeds from issuance of secured debt													298,000,000
Senior Secured Notes, 2.70%, Due 2022 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	400,000,000	[4],[5]	400,000,000	[4],[5]						400,000,000	[4],[5]		400,000,000	[4],[5]
Long-term debt interest rate	2.70%												2.70%
3.90% Senior secured notes due 2042 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	485,000,000	[3],[6]	485,000,000	[3],[6]						485,000,000	[3],[6]		485,000,000	[3],[6]
Long-term debt interest rate	3.90%												3.90%
Series 1993 5.90% Due 2023 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	1,000,000	[2]											1,000,000	[2]
Long-term debt interest rate	5.90%												5.90%
Series 1993 5.90% Due 2023 | Ameren Illinois Company | Environmental Improvement And Pollution Control Revenue Bonds
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	1,000,000	[2],[7]	32,000,000	[7]						32,000,000	[7]		1,000,000	[2],[7]
Long-term debt interest rate	5.90%												5.90%
Redemptions of long-term debt						$ 32,000,000	[8]
Minimum | Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to capitalization ratio	30.00%												30.00%

[1] These bonds are mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture and 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 remain outstanding.

[2] 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.

[3] 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 until 2042.

[4] These notes are collaterally secured by mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any series of first mortgage bonds issued under the Ameren Illinois 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 until 2024.

[5] Ameren Illinois has agreed, during the life of these notes, not to optionally redeem, purchase, or otherwise retire in full its Ameren Illinois mortgage bonds; therefore, an Ameren Illinois first mortgage bond release date will not occur as long as any of these notes are outstanding.

[6] Ameren Missouri has agreed, during the life of these notes, not to optionally redeem, purchase or otherwise retire in full its first mortgage bonds. Ameren Missouri has also agreed to prevent a first mortgage bond release date from occurring as long as any of the 8.45% senior secured notes due 2039 and any of the 3.90% senior secured notes due 2042 remain outstanding.

[7] These bonds are first mortgage bonds issued by Ameren Illinois under the CILCO mortgage indenture and 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.

[8] Less than $1 million principal amount of the bonds remain outstanding after redemption.