Property and Plant, Net (Schedule of Capitalized Software) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Capitalized software costs
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ 93	$ 78	$ 71
Gross carrying value	1,021	901
Accumulated amortization	(640)	(584)
Union Electric Company | Capitalized software costs
Finite-Lived Intangible Assets [Line Items]
Amortization expense	44	30	24
Gross carrying value	398	303
Accumulated amortization	(189)	(153)
Ameren Illinois Company | Capitalized software costs
Finite-Lived Intangible Assets [Line Items]
Amortization expense	45	45	$ 44
Gross carrying value	397	377
Accumulated amortization	$ (238)	$ (221)
Minimum
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	3 years
Maximum
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	10 years