Short-Term Debt and Liquidity (Short-Term Debt outstanding) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Short-term Debt [Line Items]
Short-term Debt	$ 615	$ 440
Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt	425	153
Union Electric Company
Short-term Debt [Line Items]
Short-term Debt	130	234
Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt	60	53
Line of Credit
Short-term Debt [Line Items]
Short-term Debt	465	0
Line of Credit | Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt	275	0
Line of Credit | Union Electric Company
Short-term Debt [Line Items]
Short-term Debt	130	0
Line of Credit | Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt	60	0
Commercial Paper
Short-term Debt [Line Items]
Short-term Debt	150	440
Commercial Paper | Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt	150	153
Commercial Paper | Union Electric Company
Short-term Debt [Line Items]
Short-term Debt	0	234
Commercial Paper | Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt	$ 0	$ 53