Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 821	$ 629
Weighted-average interest rate	2.68%	2.18%
Short-term Debt, Maximum Amount Outstanding During Period	$ 1,113	$ 1,295
Peak interest rate	5.00%	2.55%
Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 532	$ 431
Weighted-average interest rate	2.70%	2.23%
Short-term Debt, Maximum Amount Outstanding During Period	$ 651	$ 543
Peak interest rate	3.80%	2.45%
Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 141	$ 81
Weighted-average interest rate	2.73%	1.94%
Short-term Debt, Maximum Amount Outstanding During Period	$ 549	$ 481
Peak interest rate	2.97%	2.42%
Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 147	$ 117
Weighted-average interest rate	2.58%	2.21%
Short-term Debt, Maximum Amount Outstanding During Period	$ 310	$ 442
Peak interest rate	5.00%	2.55%