Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	$ 263	$ 233
Net Realized and Unrealized Gain (Loss) on Trading Securities	90	1
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	101	80
Net Realized and Unrealized Gain (Loss) on Trading Securities	90	1
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	143	147
Net Realized and Unrealized Gain (Loss) on Trading Securities	$ 0	$ 0