Short-Term Debt and Liquidity (Short-Term Debt Activity) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 649	$ 628
Weighted-average interest rate	0.24%	1.89%
Short-term Debt, Maximum Amount Outstanding During Period	$ 916	$ 908
Peak interest rate	0.33%	5.05%
Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 454	$ 157
Weighted-average interest rate	0.25%	1.94%
Short-term Debt, Maximum Amount Outstanding During Period	$ 650	$ 425
Peak interest rate	0.33%	3.30%
Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 99	$ 395
Weighted-average interest rate	0.22%	1.86%
Short-term Debt, Maximum Amount Outstanding During Period	$ 206	$ 573
Peak interest rate	0.25%	5.05%
Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 96	$ 76
Weighted-average interest rate	0.21%	1.99%
Short-term Debt, Maximum Amount Outstanding During Period	$ 353	$ 150
Peak interest rate	0.25%	3.40%