Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 815	$ 728
Weighted-average interest rate	2.85%	1.90%
Short-term Debt, Maximum Amount Outstanding During Period	$ 1,113	$ 960
Peak interest rate	3.10%	2.55%
Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 480	$ 378
Weighted-average interest rate	2.87%	1.90%
Short-term Debt, Maximum Amount Outstanding During Period	$ 618	$ 454
Peak interest rate	3.10%	2.35%
Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 246	$ 213
Weighted-average interest rate	2.84%	1.88%
Short-term Debt, Maximum Amount Outstanding During Period	$ 549	$ 282
Peak interest rate	2.97%	2.40%
Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 89	$ 137
Weighted-average interest rate	2.76%	1.96%
Short-term Debt, Maximum Amount Outstanding During Period	$ 130	$ 238
Peak interest rate	2.90%	2.55%