Short-Term Debt And Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 579	$ 668
Short-term debt	$ 597	$ 484
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.26%	1.31%
Peak short-term borrowings	$ 1,295	$ 948
Peak short-term borrowings interest rate	3.10%	2.00%
Parent Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 410	$ 573
Short-term debt	$ 470	$ 383
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.31%	1.30%
Peak short-term borrowings	$ 543	$ 841
Peak short-term borrowings interest rate	3.10%	1.90%
Union Electric Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 61	$ 5
Short-term debt	$ 55	$ 39
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.94%	1.24%
Peak short-term borrowings	$ 481	$ 64
Peak short-term borrowings interest rate	2.80%	1.78%
Ameren Illinois Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 108	$ 90
Short-term debt	$ 72	$ 62
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.26%	1.35%
Peak short-term borrowings	$ 442	$ 469
Peak short-term borrowings interest rate	2.85%	2.00%