Long-Term debt and Equity Financings (Details) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended		3 Months Ended
	Aug. 31, 2022 USD ($)	Apr. 30, 2022 USD ($) $ / shares shares	Mar. 31, 2022 USD ($) $ / shares shares	Sep. 30, 2021 USD ($) shares	Mar. 31, 2021 USD ($) shares	May 31, 2021 USD ($)
Long-Term Debt And Equity Financings [Line Items]
Shares issued under the DRPlus and 401(k) plan | shares			0.1		0.1
Stock Issued During Period, Shares, Other | shares			0.4	0.4	0.5
Stock Issued During Period, Value, Other				$ 31.0
Maximum Value Of Shares To Be Issued Under ATM Program						$ 750.0
Value of Shares Available to be Issued Under ATM Program			$ 320.0
Forward Contract Indexed to Issuer's Equity, Basis Spread			75
Issuances of common stock			$ 5.0		$ 125.0
Minimum
Long-Term Debt And Equity Financings [Line Items]
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share | $ / shares			$ 86.35
Maximum
Long-Term Debt And Equity Financings [Line Items]
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share | $ / shares			88.81
Weighted Average
Long-Term Debt And Equity Financings [Line Items]
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share | $ / shares			$ 87.61
Dividend reinvestment and 401 (k) plans
Long-Term Debt And Equity Financings [Line Items]
Accrued Proceeds from Issuance of Common Stock			$ 8.0
Issuances of common stock			$ 5.0
Forward Sale Agreements Outstanding
Long-Term Debt And Equity Financings [Line Items]
Forward Contract Indexed to Issuer's Equity, Settlement Alternatives, Shares, at Fair Value | shares			3.2
Forward Contract Indexed to Issuer's Equity, Settlement Alternatives, Cash, at Fair Value			$ 276.0
Period End Net Cash Settlement Price			21.0
Period End Net Share Settlement Price			0.2
Forward Sale Agreement Equity Offering Shares			3.2
Forward Sale Agreement Gross Sales Price			280.0
Forward Sale Agreement, Compensation Received by Counterparty			3.0
Subsequent Event | Forward Sale Agreements Outstanding
Long-Term Debt And Equity Financings [Line Items]
Forward Contract Indexed to Issuer's Equity, Indexed Shares (in shares) | shares		3.4
Subsequent Event | April 1, 2022 Forward Sale
Long-Term Debt And Equity Financings [Line Items]
Forward Contract Indexed to Issuer's Equity, Indexed Shares (in shares) | shares		0.2
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share | $ / shares		$ 91.41
Other Paid-in Capital
Long-Term Debt And Equity Financings [Line Items]
Shares issued under the DRPlus and 401(k) plan			13.0		$ 12.0
Union Electric Company | Secured Debt | Senior Secured Notes 3.90% Due 2052 | Subsequent Event
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount		$ 525.0
Debt Instrument, Interest Rate, Stated Percentage		3.90%
Proceeds from Issuance of Secured Debt		$ 519.0
Ameren Transmission Company of Illinois | Unsecured Debt | Senior Unsecured Notes, 2.96%, Due 2052
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount			$ 95.0
Debt Instrument, Interest Rate, Stated Percentage			2.96%
Ameren Transmission Company of Illinois | Unsecured Debt | Senior Unsecured Notes, 2.96%, Due 2052 | Subsequent Event
Long-Term Debt And Equity Financings [Line Items]
Proceeds from Issuance of Debt	$ 95.0
Ameren Transmission Company of Illinois | Unsecured Debt | Senior Unsecured Notes, 3.43%, Due 2050
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			3.43%
Long-Term Debt, Maturity, Year One			$ 50.0