Property And Plant, Net Property and Plant, Net (Schedule of Capitalized Software) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Capitalized Computer Software, Amortization	$ 58	$ 52	$ 47
Capitalized Computer Software, Gross	655	622
Capitalized Computer Software, Accumulated Amortization	$ (466)	(408)
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	$ 20	17	16
Capitalized Computer Software, Gross	191	178
Capitalized Computer Software, Accumulated Amortization	(107)	(87)
Ameren Illinois Company
Property, Plant and Equipment [Line Items]
Capitalized Computer Software, Amortization	36	33	$ 27
Capitalized Computer Software, Gross	241	225
Capitalized Computer Software, Accumulated Amortization	$ (146)	$ (110)

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 985
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128462-111756

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

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Amount of expense for amortization of capitalized computer software costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 985
-SubTopic 20
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128487-111756

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 985
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128462-111756

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Amount before accumulated amortization of capitalized costs for computer software, including but not limited to, acquired and internally developed computer software.

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