Supplemental Information (Inventories) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Public Utilities, Inventory [Line Items]
Fuel	$ 126	$ 123
Gas stored underground	63	71
Other materials and supplies	305	289
Total Inventories	494	483
Union Electric Company
Public Utilities, Inventory [Line Items]
Fuel	126	123
Gas stored underground	6	7
Other materials and supplies	241	228
Total Inventories	373	358
Ameren Illinois Company
Public Utilities, Inventory [Line Items]
Fuel	0	0
Gas stored underground	57	64
Other materials and supplies	64	61
Total Inventories	$ 121	$ 125