Short-Term Debt and Liquidity (Short-Term Debt Activity) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Line of Credit | Missouri Credit Agreement
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 13
Weighted-average interest rate	2.33%
Short-term Debt, Maximum Amount Outstanding During Period	$ 230
Peak interest rate	3.33%
Line of Credit | Missouri Credit Agreement | Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 1
Weighted-average interest rate	2.06%
Short-term Debt, Maximum Amount Outstanding During Period	$ 100
Peak interest rate	2.06%
Line of Credit | Missouri Credit Agreement | Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 12
Weighted-average interest rate	2.36%
Short-term Debt, Maximum Amount Outstanding During Period	$ 130
Peak interest rate	3.33%
Line of Credit | Missouri Credit Agreement | Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 0
Weighted-average interest rate	0.00%
Short-term Debt, Maximum Amount Outstanding During Period	$ 0
Peak interest rate	0.00%
Line of Credit | Illinois Credit Agreement
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 7
Weighted-average interest rate	2.06%
Short-term Debt, Maximum Amount Outstanding During Period	$ 235
Peak interest rate	2.06%
Line of Credit | Illinois Credit Agreement | Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 2
Weighted-average interest rate	2.06%
Short-term Debt, Maximum Amount Outstanding During Period	$ 175
Peak interest rate	2.06%
Line of Credit | Illinois Credit Agreement | Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 0
Weighted-average interest rate	0.00%
Short-term Debt, Maximum Amount Outstanding During Period	$ 0
Peak interest rate	0.00%
Line of Credit | Illinois Credit Agreement | Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 5
Weighted-average interest rate	2.05%
Short-term Debt, Maximum Amount Outstanding During Period	$ 60
Peak interest rate	2.05%
Commercial Paper
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 608	$ 815
Weighted-average interest rate	1.88%	2.85%
Short-term Debt, Maximum Amount Outstanding During Period	$ 854	$ 1,113
Peak interest rate	5.05%	3.10%
Commercial Paper | Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 154	$ 480
Weighted-average interest rate	1.94%	2.87%
Short-term Debt, Maximum Amount Outstanding During Period	$ 225	$ 618
Peak interest rate	3.30%	3.10%
Commercial Paper | Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 383	$ 246
Weighted-average interest rate	1.84%	2.84%
Short-term Debt, Maximum Amount Outstanding During Period	$ 521	$ 549
Peak interest rate	5.05%	2.97%
Commercial Paper | Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 71	$ 89
Weighted-average interest rate	1.99%	2.76%
Short-term Debt, Maximum Amount Outstanding During Period	$ 137	$ 130
Peak interest rate	3.40%	2.90%