Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Federal statutory corporate income tax rate:		35.00%	35.00%	35.00%
State corporate income tax rate		6.00%	4.00%	5.00%
Deferred Tax Liabilities, Net		$ 2,506	$ 4,264
Minimum
Income Taxes [Line Items]
Regulatory Liability, Amortization Period		35 years
Maximum
Income Taxes [Line Items]
Regulatory Liability, Amortization Period		60 years
Other
Income Taxes [Line Items]
Deferred Tax Liabilities, Net		$ (165)	$ (380)
Union Electric Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate:		35.00%	35.00%	35.00%
State corporate income tax rate		4.00%	3.00%	3.00%
Deferred Tax Liabilities, Net		$ 1,650	$ 3,013
Ameren Illinois Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate:		35.00%	35.00%	35.00%
State corporate income tax rate		6.00%	5.00%	5.00%
Deferred Tax Liabilities, Net		$ 1,021	$ 1,631
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 | Ameren Illinois Company
Income Taxes [Line Items]
Increase (Decrease) in Income Taxes		1
Deferred Tax Liabilities, Net		$ 79
Subsequent Event [Member]
Income Taxes [Line Items]
Federal statutory corporate income tax rate:	21.00%