Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 629	$ 754
Weighted-average interest rate	2.18%	1.27%
Short-term Debt, Maximum Amount Outstanding During Period	$ 1,295	$ 948
Peak interest rate	2.55%	1.50%
Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 431	$ 669
Weighted-average interest rate	2.23%	1.27%
Short-term Debt, Maximum Amount Outstanding During Period	$ 543	$ 841
Peak interest rate	2.45%	1.50%
Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 81	$ 7
Weighted-average interest rate	1.94%	1.20%
Short-term Debt, Maximum Amount Outstanding During Period	$ 481	$ 64
Peak interest rate	2.42%	1.41%
Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 117	$ 78
Weighted-average interest rate	2.21%	1.28%
Short-term Debt, Maximum Amount Outstanding During Period	$ 442	$ 193
Peak interest rate	2.55%	1.50%