Short-Term Debt and Liquidity (Short-Term Debt Activity) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Short-term Debt [Line Items]
Average daily amount outstanding	$ 789	$ 649
Weighted-average interest rate	0.41%	0.24%
Short-term Debt, Maximum Amount Outstanding During Period	$ 1,101	$ 916
Peak interest rate	1.15%	0.33%
Parent Company
Short-term Debt [Line Items]
Average daily amount outstanding	$ 282	$ 454
Weighted-average interest rate	0.41%	0.25%
Short-term Debt, Maximum Amount Outstanding During Period	$ 466	$ 650
Peak interest rate	1.05%	0.33%
Union Electric Company
Short-term Debt [Line Items]
Average daily amount outstanding	$ 408	$ 99
Weighted-average interest rate	0.44%	0.22%
Short-term Debt, Maximum Amount Outstanding During Period	$ 539	$ 206
Peak interest rate	1.06%	0.25%
Ameren Illinois Company
Short-term Debt [Line Items]
Average daily amount outstanding	$ 99	$ 96
Weighted-average interest rate	0.33%	0.21%
Short-term Debt, Maximum Amount Outstanding During Period	$ 142	$ 353
Peak interest rate	1.15%	0.25%