Summary Of Significant Accounting Policies Summary of Significant Account Policies (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2017
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	$ 233	$ 175
Net Realized and Unrealized Gain (Loss) on Trading Securities	1	36
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	80	61
Net Realized and Unrealized Gain (Loss) on Trading Securities	1	$ 36
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	$ 147		$ 79