Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 693	$ 808
Weighted-average interest rate	2.12%	1.19%
Short-term Debt, Maximum Amount Outstanding During Period	$ 1,295	$ 948
Peak interest rate	2.55%	1.50%
Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 397	$ 736
Weighted-average interest rate	2.14%	1.19%
Short-term Debt, Maximum Amount Outstanding During Period	$ 506	$ 841
Peak interest rate	2.45%	1.50%
Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 123	$ 6
Weighted-average interest rate	1.94%	1.10%
Short-term Debt, Maximum Amount Outstanding During Period	$ 481	$ 60
Peak interest rate	2.42%	1.41%
Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 174	$ 66
Weighted-average interest rate	2.20%	1.14%
Short-term Debt, Maximum Amount Outstanding During Period	$ 442	$ 163
Peak interest rate	2.55%	1.50%