Supplemental Information (Inventories) (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Public Utilities, Inventory [Line Items]
Fuel	$ 118	$ 115
Natural gas stored underground	99	57
Materials, supplies, and other	375	349
Total inventories	592	521
Union Electric Company
Public Utilities, Inventory [Line Items]
Fuel	118	115
Natural gas stored underground	9	5
Materials, supplies, and other	292	266
Total inventories	419	386
Ameren Illinois Company
Public Utilities, Inventory [Line Items]
Fuel	0	0
Natural gas stored underground	90	52
Materials, supplies, and other	83	83
Total inventories	$ 173	$ 135