Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 822	$ 693
Weighted-average interest rate	2.79%	2.12%
Short-term Debt, Maximum Amount Outstanding During Period	$ 1,113	$ 1,295
Peak interest rate	3.10%	2.55%
Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 542	$ 397
Weighted-average interest rate	2.80%	2.14%
Short-term Debt, Maximum Amount Outstanding During Period	$ 636	$ 506
Peak interest rate	3.10%	2.45%
Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 174	$ 123
Weighted-average interest rate	2.79%	1.94%
Short-term Debt, Maximum Amount Outstanding During Period	$ 549	$ 481
Peak interest rate	2.97%	2.42%
Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 106	$ 174
Weighted-average interest rate	2.72%	2.20%
Short-term Debt, Maximum Amount Outstanding During Period	$ 202	$ 442
Peak interest rate	2.90%	2.55%