Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	$ 208	$ 202
Net Realized and Unrealized Gain (Loss) on Trading Securities	64	(11)
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	92	73
Net Realized and Unrealized Gain (Loss) on Trading Securities	64	(11)
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	$ 106	$ 114