Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 728	$ 736
Weighted-average interest rate	1.90%	1.06%
Peak commercial paper during period	$ 960	$ 914
Peak interest rate	2.55%	1.30%
Ameren (parent)
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 378	$ 682
Weighted-average interest rate	1.90%	1.07%
Peak commercial paper during period	$ 454	$ 810
Peak interest rate	2.35%	1.30%
Union Electric Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 213	$ 4
Weighted-average interest rate	1.88%	0.93%
Peak commercial paper during period	$ 282	$ 45
Peak interest rate	2.40%	1.15%
Ameren Illinois Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 137	$ 50
Weighted-average interest rate	1.96%	0.95%
Peak commercial paper during period	$ 238	$ 74
Peak interest rate	2.55%	1.15%