Summary of Significant Accounting Policies (Cash and Cash Equivalents) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	$ 11	$ 10	$ 9	$ 9
Restricted Cash and Cash Equivalents	15	21	19	20
Restricted Cash and Cash Equivalents, Noncurrent	63	35	27	22
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	3	2	2	1
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	92	68	57	52
Union Electric Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	0	0	0	0
Restricted Cash and Cash Equivalents	4	5	4	4
Restricted Cash and Cash Equivalents, Noncurrent	0	0	0	0
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	3	2	2	1
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	7	7	6	5
Ameren Illinois Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	0	0	0	0
Restricted Cash and Cash Equivalents	8	6	5	6
Restricted Cash and Cash Equivalents, Noncurrent	63	35	27	22
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 71	$ 41	$ 32	$ 28

Restricted cash and equivalents reserved for use associated with the Callaway Energy Center.

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[Line Items] for Tabular disclosure of the components of cash and cash equivalents, including restricted amounts.

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Amount of cash and cash equivalents, and cash and cash equivalents restricted to withdrawal or usage. Excludes amount for disposal group and discontinued operations. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Amount of cash and cash equivalents restricted as to withdrawal or usage, classified as current. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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