Short-Term Debt And Liquidity Short-Term Debt And Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 668	$ 552
Short-term debt	$ 484	$ 558
Short-term Debt, Weighted Average Interest Rate	1.31%	0.80%
Peak short-term borrowings	$ 948	$ 839
Peak short-term borrowings interest rate	2.00%	1.05%
Parent Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 573	$ 440
Short-term debt	$ 383	$ 507
Short-term Debt, Weighted Average Interest Rate	1.30%	0.82%
Peak short-term borrowings	$ 841	$ 574
Peak short-term borrowings interest rate	1.90%	1.05%
Union Electric Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 5	$ 60
Short-term debt	$ 39	$ 0
Short-term Debt, Weighted Average Interest Rate	1.24%	0.74%
Peak short-term borrowings	$ 64	$ 208
Peak short-term borrowings interest rate	1.78%	0.85%
Ameren Illinois Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 90	$ 52
Short-term debt	$ 62	$ 51
Short-term Debt, Weighted Average Interest Rate	1.35%	0.69%
Peak short-term borrowings	$ 469	$ 195
Peak short-term borrowings interest rate	2.00%	0.90%