Long-Term Debt And Equity Financings (Narrative) (Details) - USD ($)		3 Months Ended		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014	Mar. 31, 2016	May 31, 2014	Oct. 31, 2013
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
Restricted Cash and Cash Equivalents		$ 8,000,000		$ 8,000,000
Repayments of Other Long-term Debt				395,000,000		$ 120,000,000		$ 697,000,000
Debt Default Provision Excess				$ 25,000,000
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
Repayments of Other Long-term Debt				$ 266,000,000		$ 120,000,000		109,000,000
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
Repayments of Other Long-term Debt				$ 129,000,000		$ 0		163,000,000
Common stock equity to capitalization ratio		51.00%		51.00%
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				$ 0		0		$ 425,000,000
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%
Senior Unsecured Notes270 due 2020 [Member] | Parent Company
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate		2.70%		2.70%
Senior Unsecured Notes270 due 2020 [Member] | Parent Company | Unsecured Debt [Member]
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 350,000,000		$ 350,000,000		350,000,000
Long-term debt interest rate		2.70%		2.70%
Proceeds from issuance of secured debt		$ 348,000,000
Senior Unsecured Notes365 due 2026 [Member] | Parent Company
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate		3.65%		3.65%
Senior Unsecured Notes365 due 2026 [Member] | Parent Company | Unsecured Debt [Member]
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 350,000,000		$ 350,000,000		350,000,000
Long-term debt interest rate		3.65%		3.65%
Proceeds from issuance of secured debt				$ 347,000,000
5.40% Senior secured notes due 2016 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 0	[1]	$ 0	[1]	260,000,000	[1]		$ 260,000,000
Long-term debt interest rate		5.40%		5.40%
Senior Secured Notes, 3.65%, Due 2045 - $150 Issuance [Member] | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 150,000,000		$ 150,000,000
Proceeds from issuance of secured debt				148,000,000
Senior Secured Notes, 3.65%, Due 2045 - $250 Issuance [Member] | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		250,000,000		250,000,000
Proceeds from issuance of secured debt				247,000,000
Senior Secured Notes, 3.65%, Due 2045 [Member] | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	[1]	$ 400,000,000		$ 400,000,000		250,000,000
Long-term debt interest rate		3.65%		3.65%
Senior Secured Notes 4.75% Due2015 [Member] | Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate		4.75%		4.75%
Repayments of Other Long-term Debt				$ 114,000,000
Senior Secured Notes 6.20% Due 2016 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 0		$ 0		54,000,000
Long-term debt interest rate		6.20%		6.20%
Repayments of Other Long-term Debt				$ 54,000,000
Senior Secured Notes 6.25% Due 2016 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 0		$ 0		75,000,000
Long-term debt interest rate		6.25%		6.25%
Repayments of Other Long-term Debt				$ 75,000,000
Senior Secured Notes, 4.15%, Due 2046 - $240 Issuance [Member] | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 240,000,000		240,000,000
Proceeds from issuance of secured debt		245,000,000
Senior Secured Notes, 4.15%, Due 2046 - $250 Issuance [Member] | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		250,000,000		250,000,000
Proceeds from issuance of secured debt		245,000,000
Senior Secured Notes, 4.15%, Due 2046 [Member] | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	[2]	$ 490,000,000		$ 490,000,000		$ 250,000,000
Long-term debt interest rate		4.15%		4.15%
Minimum | Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to capitalization ratio		30.00%		30.00%

[1] (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.

[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.