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Long-Term Debt And Equity Financings (Narrative) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized (in shares)		100,000,000
Stock Issued During Period, Shares, New Issues		1,200,000	0	0
Stock Issued During Period, Shares, Other		700,000	0	0
Stock Issued		$ 35,000,000
Common Stock, Shares Authorized Under 401(k) Plan		4,000,000
Proceeds from Issuance of Common Stock		$ 74,000,000	$ 0	$ 0
Preferred stock, par value (in dollars per share)		$ 0.01
Preferred stock, shares outstanding (in shares)		0
Common stock, shares authorized		6,000,000
Restricted Cash and Cash Equivalents		$ 1,000,000	8,000,000
Repayments of Other Long-term Debt		841,000,000	681,000,000	395,000,000
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
Preferred stock, par value (in dollars per share)		$ 1
Repayments of Other Long-term Debt		$ 384,000,000	431,000,000	266,000,000
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized (in shares)		2,600,000.0
Repayments of Other Long-term Debt		$ 457,000,000	250,000,000	$ 129,000,000
Common stock equity to capitalization ratio		51.00%
Ameren Missouri and Ameren Illinois
Long-Term Debt And Equity Financings [Line Items]
Bonds interest rate assumption		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%
Proceeds from issuance of secured debt		$ 396,000,000
6.40% Senior secured notes due 2017 \| Union Electric Company \| Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate		6.40%
Repayments of Other Long-term Debt		$ 425,000,000
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
Long-term debt interest rate		3.65%
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	500,000,000
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 \| Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate	6.125%
Repayments of Other Long-term Debt	$ 250,000,000
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
Long-term debt interest rate		4.15%
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%
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	450,000,000
Long-term debt interest rate		3.43%
Proceeds from Issuance of Unsecured Debt		$ 449,000,000
Ratio of Indebtedness to Net Capital		0.70
Ratio of Indebtedness to Total Assets		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
Long-term debt interest rate		5.90%
First Mortgage Bonds, 4.00%, Due 2048 - $425 Issuance [Member] \| Union Electric Company \| Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 425,000,000	0
Long-term debt interest rate		4.00%
Proceeds from issuance of secured debt		$ 419,000,000
6.00% Senior secured notes due 2018 \| Union Electric Company \| Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 0	179,000,000
Long-term debt interest rate		6.00%
Repayments of Other Long-term Debt		$ 179,000,000
Senior Secured Notes 5.10% Due 2018 [Member] \| Union Electric Company \| Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate		5.10%
Repayments of Other Long-term Debt		$ 199,000,000
First Mortgage Bonds, 3.80%, Due 2028 [Member] \| Ameren Illinois Company \| Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 430,000,000	0
Long-term debt interest rate		3.80%
Proceeds from issuance of secured debt		$ 427,000,000
Senior Secured Notes 6.25% Due 2018 \| Ameren Illinois Company \| Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 0	144,000,000
Long-term debt interest rate		6.25%
Repayments of Other Long-term Debt		$ 144,000,000
First Mortgage Bonds, 4.50%, Due 2049 [Member] \| Ameren Illinois Company \| Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 500,000,000	0
Long-term debt interest rate		4.50%
Proceeds from issuance of secured debt		$ 495,000,000
Senior Secured Notes 9.75% Due 2018 \| Ameren Illinois Company \| Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 0	$ 313,000,000
Long-term debt interest rate		9.75%
Repayments of Other Long-term Debt		$ 313,000,000
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
Minimum \| Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to capitalization ratio		30.00%
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