Short-Term Debt And Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Short-term Debt [Line Items]
Average daily amount outstanding	$ 263	$ 700
Commercial paper issuances outstanding at period-end	$ 490	$ 440
Weighted-average interest rate	1.31%	2.60%
Peak amount outstanding during period(a)	$ 908	$ 1,113
Peak interest rate	5.05%	5.00%
Parent Company
Short-term Debt [Line Items]
Average daily amount outstanding	$ 108	$ 421
Commercial paper issuances outstanding at period-end	$ 490	$ 153
Weighted-average interest rate	1.04%	2.66%
Peak amount outstanding during period(a)	$ 490	$ 651
Peak interest rate	3.30%	3.80%
Union Electric Company
Short-term Debt [Line Items]
Average daily amount outstanding	$ 109	$ 122
Commercial paper issuances outstanding at period-end	$ 0	$ 234
Weighted-average interest rate	1.73%	2.62%
Peak amount outstanding during period(a)	$ 573	$ 549
Peak interest rate	5.05%	2.97%
Ameren Illinois Company
Short-term Debt [Line Items]
Average daily amount outstanding	$ 46	$ 157
Commercial paper issuances outstanding at period-end	$ 0	$ 53
Weighted-average interest rate	0.97%	2.43%
Peak amount outstanding during period(a)	$ 250	$ 356
Peak interest rate	3.40%	5.00%