Summary Of Significant Accounting Policies Summary of Significant Account Policies (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018		Mar. 31, 2017
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	$ 202		$ 164
Available-for-sale Securities, Gross Realized Gains	1	[1]	4
Unrealized Gain (Loss) on Investments	(11)		19
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	73		50
Available-for-sale Securities, Gross Realized Gains	1	[1]	4
Unrealized Gain (Loss) on Investments	(11)		19
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	114		74
Nuclear Fuel
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	1	[1]	13
Nuclear Fuel | Union Electric Company
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	$ 1	[1]	$ 13

[1] Less than $1 million.