Long-Term Debt And Equity Financings (Narrative) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Mar. 31, 2017	Oct. 31, 2013
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized (in shares)	100,000,000	100,000,000
Preferred stock, par value (in dollars per share)	$ 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		681,000,000	$ 395,000,000	$ 120,000,000
Debt Default Provision Excess		$ 25,000,000
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized (in shares)	7,500,000.0	7,500,000.0
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares authorized	150,000,000	150,000,000	150,000,000
Repayments of Other Long-term Debt		$ 431,000,000	$ 266,000,000	120,000,000
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized (in shares)	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		$ 250,000,000	$ 129,000,000	$ 0
Common stock equity to capitalization ratio	51.00%	51.00%
Ameren Transmission Company of Illinois
Long-Term Debt And Equity Financings [Line Items]
Redemption price, percentage		100.00%
Parent Company
Long-Term Debt And Equity Financings [Line Items]
Common stock, shares authorized	6,000,000	6,000,000				4,000,000
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 Secured Notes, 2.95%, Due 2027 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 400,000,000	$ 400,000,000
Long-term debt interest rate	2.95%	2.95%
Proceeds from issuance of secured debt		$ 396,000,000
6.40% Senior secured notes due 2017 | Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate	6.40%	6.40%
Repayments of Other Long-term Debt		$ 425,000,000
6.40% Senior secured notes due 2017 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 0	$ 0	425,000,000
Long-term debt interest rate	6.40%	6.40%
5.40% Senior secured notes due 2016 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount					$ 260,000,000
Long-term debt interest rate	5.40%	5.40%
Senior Secured Notes, 3.65%, Due 2045 - $150 Issuance | 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 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 400,000,000	$ 400,000,000	400,000,000
Long-term debt interest rate	3.65%	3.65%
Senior Secured Notes 4.75% Due2015 | 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, 3.70%, Due 2047 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 500,000,000	$ 500,000,000	0
Long-term debt interest rate	3.70%	3.70%
Proceeds from issuance of secured debt		$ 492,000,000
Senior Secured Notes6125 Due2017 | Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate	6.125%	6.125%
Repayments of Other Long-term Debt		$ 250,000,000
Senior Secured Notes6125 Due2017 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 0	$ 0	250,000,000
Long-term debt interest rate	6.125%	6.125%
Senior Secured Notes 6.20% Due 2016 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
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]
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 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 240,000,000	240,000,000
Senior Secured Notes, 4.15%, Due 2046 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 490,000,000	$ 490,000,000	490,000,000
Long-term debt interest rate	4.15%	4.15%
Proceeds from issuance of secured debt	$ 245,000,000
Senior Unsecured Notes, 3.43%, Due 2050 | Ameren Transmission Company of Illinois | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate	3.43%	3.43%
Senior Unsecured Notes, 3.43%, Due 2050 | Ameren Transmission Company of Illinois | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 300,000,000	$ 300,000,000	$ 0
Long-term debt interest rate	3.43%	3.43%
Proceeds from Issuance of Unsecured Debt		$ 449,000,000
Ratio of Indebtedness to Net Capital	0.70	0.70
Ratio of Indebtedness to Total Assets	0.10	0.10
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	$ 1,000,000
Long-term debt interest rate	5.90%	5.90%
Maximum | Senior Unsecured Notes, 3.43%, Due 2050 | Ameren Transmission Company of Illinois | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 450,000,000	$ 450,000,000
Minimum | Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to capitalization ratio	30.00%	30.00%
Minimum | Ameren Transmission Company of Illinois
Long-Term Debt And Equity Financings [Line Items]
Redemption price, percentage		5.00%
Minimum | Senior Unsecured Notes, 3.43%, Due 2050 | Ameren Transmission Company of Illinois | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 150,000,000	$ 150,000,000