Short-Term Debt And Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 700	$ 579
Short-term debt	$ 440	$ 597
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.60%	2.26%
Short-term Debt, Maximum Amount Outstanding During Period	$ 1,113	$ 1,295
Peak short-term borrowings interest rate	5.00%	3.10%
Parent Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 421	$ 410
Short-term debt	$ 153	$ 470
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.66%	2.31%
Short-term Debt, Maximum Amount Outstanding During Period	$ 651	$ 543
Peak short-term borrowings interest rate	3.80%	3.10%
Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 122	$ 61
Short-term debt	$ 234	$ 55
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.62%	1.94%
Short-term Debt, Maximum Amount Outstanding During Period	$ 549	$ 481
Peak short-term borrowings interest rate	2.97%	2.80%
Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 157	$ 108
Short-term debt	$ 53	$ 72
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.43%	2.26%
Short-term Debt, Maximum Amount Outstanding During Period	$ 356	$ 442
Peak short-term borrowings interest rate	5.00%	2.85%