Income Taxes Income Taxes (Schedule of Remeasurement of Deferred Income Taxes due to the Tax Cuts and Jobs Act) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Deferred Federal Income Tax Expense (Benefit)	$ 511	$ 299	$ 299
Tax Cuts and Jobs Act [Member]
Income Taxes [Line Items]
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Accumulated Deferred Income Taxes	(2,253)
Deferred Federal Income Tax Expense (Benefit)	154
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Regulatory Assets	(114)
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Regulatory Liabilities	2,293
Union Electric Company
Income Taxes [Line Items]
Deferred Federal Income Tax Expense (Benefit)	76	161	71
Union Electric Company | Tax Cuts and Jobs Act [Member]
Income Taxes [Line Items]
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Accumulated Deferred Income Taxes	(1,419)
Deferred Federal Income Tax Expense (Benefit)	32
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Regulatory Assets	(89)
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Regulatory Liabilities	1,362
Ameren Illinois Company
Income Taxes [Line Items]
Deferred Federal Income Tax Expense (Benefit)	185	117	193
Ameren Illinois Company | Tax Cuts and Jobs Act [Member]
Income Taxes [Line Items]
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Accumulated Deferred Income Taxes	(871)
Deferred Federal Income Tax Expense (Benefit)	(5)
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Regulatory Assets	(24)
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Regulatory Liabilities	842
Other
Income Taxes [Line Items]
Deferred Federal Income Tax Expense (Benefit)	250	$ 21	$ 35
Other | Tax Cuts and Jobs Act [Member]
Income Taxes [Line Items]
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Accumulated Deferred Income Taxes	37
Deferred Federal Income Tax Expense (Benefit)	127
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Regulatory Assets	(1)
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Regulatory Liabilities	$ 89