Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Federal statutory corporate income tax rate:	21.00%	35.00%	35.00%
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	$ 13
Deferred Tax Liabilities, Net	$ 2,623	$ 2,506
Minimum
Income Taxes [Line Items]
Regulatory Liability, Amortization Period	30 years
Maximum
Income Taxes [Line Items]
Regulatory Liability, Amortization Period	60 years
Parent Company
Income Taxes [Line Items]
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	$ 5
Other
Income Taxes [Line Items]
Deferred Tax Liabilities, Net	$ (30)	$ (165)
Union Electric Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate:	21.00%	35.00%	35.00%
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	$ 4
Deferred Tax Liabilities, Net	$ 1,534	$ 1,650
Ameren Illinois Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate:	21.00%	35.00%	35.00%
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	$ 4
Deferred Tax Liabilities, Net	$ 1,119	$ 1,021
State
Income Taxes [Line Items]
State Income Tax Statutory Rate		7.75%
Deferred Tax Liabilities, Net		$ 97
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 | Other
Income Taxes [Line Items]
Increase (Decrease) in Income Taxes		$ 14
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]
Deferred Tax Liabilities, Net		79
Increase (Decrease) in Income Taxes		$ 1