Short-Term Debt and Liquidity (Short-Term Debt Activity) (Details) - Commercial Paper - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 335	$ 822
Weighted-average interest rate	1.92%	2.79%
Short-term Debt, Maximum Amount Outstanding During Period	$ 908	$ 1,113
Peak interest rate	5.05%	3.10%
Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 93	$ 542
Weighted-average interest rate	2.05%	2.80%
Short-term Debt, Maximum Amount Outstanding During Period	$ 425	$ 636
Peak interest rate	3.30%	3.10%
Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 202	$ 174
Weighted-average interest rate	1.86%	2.79%
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	$ 40	$ 106
Weighted-average interest rate	1.98%	2.72%
Short-term Debt, Maximum Amount Outstanding During Period	$ 150	$ 202
Peak interest rate	3.40%	2.90%