Short-Term Debt and Liquidity (Short-Term Debt Activity) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Short-term Debt [Line Items]
Average daily amount outstanding	$ 782	$ 335
Weighted-average interest rate	0.23%	1.92%
Short-term Debt, Maximum Amount Outstanding During Period	$ 1,134	$ 908
Peak interest rate	0.33%	5.05%
Ameren (parent)
Short-term Debt [Line Items]
Average daily amount outstanding	$ 388	$ 93
Weighted-average interest rate	0.24%	2.05%
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]
Average daily amount outstanding	$ 183	$ 202
Weighted-average interest rate	0.22%	1.86%
Short-term Debt, Maximum Amount Outstanding During Period	$ 546	$ 573
Peak interest rate	0.25%	5.05%
Ameren Illinois Company
Short-term Debt [Line Items]
Average daily amount outstanding	$ 211	$ 40
Weighted-average interest rate	0.22%	1.98%
Short-term Debt, Maximum Amount Outstanding During Period	$ 485	$ 150
Peak interest rate	0.25%	3.40%