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- Definition

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

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+ References

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(e))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Related Parties
-URI http://asc.fasb.org/extlink&oid=16382449

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07.1(c))
-URI http://asc.fasb.org/extlink&oid=6488393&loc=d3e606610-122999

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Related Party Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Operating Revenues	$ 0	$ 0	$ 0
Union Electric Company \| Ameren Missouri Power Supply Agreements with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues	23	28	15
Union Electric Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	26	25	25
Union Electric Company \| Ameren Illinois Transmission Services Agreement with Marketing Company [Member]
Related Party Transaction [Line Items]
Operating Revenues	1
Union Electric Company \| Ameren Missouri and Ameren Illinois miscellaneous support services
Related Party Transaction [Line Items]
Operating Revenues		1	2
Union Electric Company \| Total Related Party Operating Revenues [Member]
Related Party Transaction [Line Items]
Operating Revenues	49	54	42
Union Electric Company \| Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	149	129	131
Union Electric Company \| Money Pool
Related Party Transaction [Line Items]
Interest (Charges) Income	1	1	1
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	0	0	0
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	4	5	4
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois miscellaneous support services
Related Party Transaction [Line Items]
Operating Revenues	1	1	1
Ameren Illinois Company \| Total Related Party Operating Revenues [Member]
Related Party Transaction [Line Items]
Operating Revenues	5	5	4
Ameren Illinois Company \| Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Operating Expenses	23	28	15
Ameren Illinois Company \| Ameren Illinois transmission agreements with ATXI
Related Party Transaction [Line Items]
Operating Expenses	2	2	2
Ameren Illinois Company \| Purchased Power
Related Party Transaction [Line Items]
Operating Expenses	25	30	17
Ameren Illinois Company \| Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	139	123	119
Ameren Illinois Company \| Money Pool
Related Party Transaction [Line Items]
Interest (Charges) Income	$ 1	$ 1	$ 1