Long-Term debt and Equity Financings (Narrative) (Details) - USD ($) shares in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Feb. 11, 2021
Long-Term Debt And Equity Financings [Line Items]
Stock Issued During Period, Shares, New Issues	0.1	0.2
Issuance of common stock	$ 125,000,000	$ 13,000,000
Stock Issued During Period, Shares, Other	0.5	0.5
Issuance of common stock for stock-based compensation	$ 33,000,000	$ 38,000,000
Shares issued under forward sale agreement (in shares)	1.6	0.0	1.6
Common Stock Value Issued Through Forward Sale Agreement			$ 113,000,000
Dividend reinvestment and 401 (k) plans
Long-Term Debt And Equity Financings [Line Items]
Issuance of common stock	$ 12,000,000
Ameren (parent) | Unsecured Debt | Senior Secured Notes 1.75% Due 2028 [Domain]
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 450,000,000
Debt Instrument, Interest Rate, Stated Percentage	1.75%
Proceeds from Issuance of Debt	$ 447,000,000
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Issuance of common stock for stock-based compensation	0	$ 0
Capital contributions from parent	113,000,000	0
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Issuance of common stock for stock-based compensation	0	0
Capital contributions from parent	40,000,000	100,000,000
Redemption of preferred stock	$ (13,000,000)	$ 0
Ameren Illinois Company | Series6625
Long-Term Debt And Equity Financings [Line Items]
Preferred Stock, Dividend Rate, Percentage	6.625%
Redemption of preferred stock	$ (12,000,000)
Ameren Illinois Company | Series775
Long-Term Debt And Equity Financings [Line Items]
Preferred Stock, Dividend Rate, Percentage	7.75%
Redemption of preferred stock	$ (1,000,000)