Property and Plant, Net (Schedule of Capitalized Software) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Capitalized Computer Software, Amortization	$ 71	$ 58	$ 52
Capitalized Computer Software, Gross	734	655
Capitalized Computer Software, Accumulated Amortization	$ (514)	(466)
Software and Software Development Costs [Member] \| Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	10 years
Software and Software Development Costs [Member] \| Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Union Electric Company
Property, Plant and Equipment [Line Items]
Capitalized Computer Software, Amortization	$ 24	20	17
Capitalized Computer Software, Gross	223	191
Capitalized Computer Software, Accumulated Amortization	(125)	(107)
Ameren Illinois Company
Property, Plant and Equipment [Line Items]
Capitalized Computer Software, Amortization	44	36	$ 33
Capitalized Computer Software, Gross	297	241
Capitalized Computer Software, Accumulated Amortization	$ (183)	$ (146)

For each balance sheet presented, the amount of accumulated amortization for capitalized computer software costs.

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Amount of expense for amortization of capitalized computer software costs.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 985
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