Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes [Line Items]
Federal statutory corporate income tax rate:	21.00%	21.00%	35.00%
Deferred Federal Income Tax Expense (Benefit)	$ 185	$ 220	$ 511
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	13
Deferred Tax Liabilities, Net	$ 2,919	$ 2,666
Minimum
Income Taxes [Line Items]
Regulatory Liability, Amortization Period	25 years
Maximum
Income Taxes [Line Items]
Regulatory Liability, Amortization Period	65 years
Union Electric Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate:	21.00%	21.00%	35.00%
Deferred Federal Income Tax Expense (Benefit)	$ 37	$ 22	$ 76
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	4
Deferred Tax Liabilities, Net	$ 1,612	$ 1,576
Ameren Illinois Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate:	21.00%	21.00%	35.00%
Deferred Federal Income Tax Expense (Benefit)	$ 66	$ 75	$ 185
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	4
Deferred Tax Liabilities, Net	1,224	1,119
Other
Income Taxes [Line Items]
Deferred Federal Income Tax Expense (Benefit)	82	123	250
Deferred Tax Liabilities, Net	$ 83	$ (29)
Tax Cuts and Jobs Act [Member]
Income Taxes [Line Items]
Deferred Federal Income Tax Expense (Benefit)			154
Tax Cuts and Jobs Act [Member] | Union Electric Company
Income Taxes [Line Items]
Deferred Federal Income Tax Expense (Benefit)			32
Tax Cuts and Jobs Act [Member] | Ameren Illinois Company
Income Taxes [Line Items]
Deferred Federal Income Tax Expense (Benefit)			$ (5)
State
Income Taxes [Line Items]
State Income Tax Statutory Rate		7.75%
State | Minimum
Income Taxes [Line Items]
State Income Tax Statutory Rate		7.30%
State | Maximum
Income Taxes [Line Items]
State Income Tax Statutory Rate		9.50%
State | Union Electric Company
Income Taxes [Line Items]
State Income Tax Statutory Rate	4.00%
Deferred Tax Liabilities, Net	$ 122
State | Union Electric Company | Maximum
Income Taxes [Line Items]
State Income Tax Statutory Rate	6.25%
State | Ameren Illinois Company
Income Taxes [Line Items]
Increase (Decrease) in Income Taxes		$ 1
State | Other
Income Taxes [Line Items]
Increase (Decrease) in Income Taxes		$ 14