Amount of debt and equity financial instruments classified as either held-to-maturity or available-for-sale.

No definition available.

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- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
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- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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- Definition

Amount of long-term debt and capital lease obligation due after one year or beyond the normal operating cycle, if longer.

+ References

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- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

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- Definition

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
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- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents the amount of short-term debt existing as of the balance sheet date.

+ References

No definition available.

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Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Financial Assets and Liabilities) (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt	$ 799	$ 597
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	118	107	$ 89	$ 68
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt	55	55
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	12	8	10	7
Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt	126	72
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	93	80	$ 52	$ 41
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities and Held-to-maturity Securities	270	270
Short-term Debt	799	597
Long-term debt (including current portion)	8,557	8,439
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	118	107
Debt Issuance Costs, Net	62	58
Carrying Amount \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities and Held-to-maturity Securities	270	270
Short-term Debt	55	55
Long-term debt (including current portion)	4,116	3,998
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	12	8
Debt Issuance Costs, Net	25	22
Carrying Amount \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt	126	72
Long-term debt (including current portion)	3,296	3,296
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	93	80
Debt Issuance Costs, Net	32	31
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	799	597
Investments, Fair Value Disclosure	270	270
Long-term Debt, Fair Value	9,070	8,669
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	118	107
Fair Value \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	118	107
Fair Value \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	799	597
Investments, Fair Value Disclosure	270	270
Long-term Debt, Fair Value	8,623	8,240
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Fair Value \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	447	429
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Fair Value \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	55	55
Investments, Fair Value Disclosure	270	270
Long-term Debt, Fair Value	4,404	4,156
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	12	8
Fair Value \| Union Electric Company \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	12	8
Fair Value \| Union Electric Company \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	55	55
Investments, Fair Value Disclosure	270	270
Long-term Debt, Fair Value	4,404	4,156
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Fair Value \| Union Electric Company \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Fair Value \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	126	72
Long-term Debt, Fair Value	3,515	3,391
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	93	80
Fair Value \| Ameren Illinois Company \| Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	93	80
Fair Value \| Ameren Illinois Company \| Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	126	72
Long-term Debt, Fair Value	3,515	3,391
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Fair Value \| Ameren Illinois Company \| Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 0	$ 0