Short-Term Debt and Liquidity (Short-Term Debt Activity) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021
Short-term Debt [Line Items]
Average daily amount outstanding	$ 702	$ 782
Weighted-average interest rate	0.75%	0.23%
Short-term Debt, Maximum Amount Outstanding During Period	$ 1,101	$ 1,134
Peak interest rate	2.05%	0.33%
Parent Company
Short-term Debt [Line Items]
Average daily amount outstanding	$ 374	$ 388
Weighted-average interest rate	0.87%	0.24%
Short-term Debt, Maximum Amount Outstanding During Period	$ 595	$ 650
Peak interest rate	2.05%	0.33%
Union Electric Company
Short-term Debt [Line Items]
Average daily amount outstanding	$ 271	$ 183
Weighted-average interest rate	0.65%	0.22%
Short-term Debt, Maximum Amount Outstanding During Period	$ 539	$ 546
Peak interest rate	2.05%	0.25%
Ameren Illinois Company
Short-term Debt [Line Items]
Average daily amount outstanding	$ 57	$ 211
Weighted-average interest rate	0.47%	0.22%
Short-term Debt, Maximum Amount Outstanding During Period	$ 142	$ 485
Peak interest rate	2.05%	0.25%