Short-Term Debt and Liquidity (Short-Term Debt Activity) (Details) - Commercial Paper - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 257	$ 821
Weighted-average interest rate	1.70%	2.68%
Short-term Debt, Maximum Amount Outstanding During Period	$ 908	$ 1,113
Peak interest rate	5.05%	5.00%
Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 62	$ 532
Weighted-average interest rate	2.04%	2.70%
Short-term Debt, Maximum Amount Outstanding During Period	$ 425	$ 651
Peak interest rate	3.30%	3.80%
Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 142	$ 141
Weighted-average interest rate	1.76%	2.73%
Short-term Debt, Maximum Amount Outstanding During Period	$ 573	$ 549
Peak interest rate	5.05%	2.97%
Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 53	$ 147
Weighted-average interest rate	1.10%	2.58%
Short-term Debt, Maximum Amount Outstanding During Period	$ 243	$ 310
Peak interest rate	3.40%	5.00%