Short-Term Debt And Liquidity (Borrowing Activity On Credit Agreements) (Detail) (2010 Missouri Credit Agreement [Member], USD $) In Millions, unless otherwise specified	6 Months Ended
Jun. 30, 2012
Short-term Debt [Line Items]
Average daily borrowings outstanding during 2012	$ 2
Outstanding credit facility borrowings at period end
Weighted-average interest rate during 2012	4.15%
Peak credit facility borrowings during 2012	50
Peak interest rate during 2012	4.15%
Ameren Corporation [Member]
Short-term Debt [Line Items]
Outstanding credit facility borrowings at period end
Ameren Missouri [Member]
Short-term Debt [Line Items]
Average daily borrowings outstanding during 2012	2
Outstanding credit facility borrowings at period end
Weighted-average interest rate during 2012	4.15%
Peak credit facility borrowings during 2012	$ 50
Peak interest rate during 2012	4.15%

Line Of Credit Facility Peak Interest Rate

No definition available.

Name:	aee_LineOfCreditFacilityPeakInterestRate
Namespace Prefix:	aee_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount borrowed under the credit facility as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

Name:	us-gaap_LineOfCreditFacilityAmountOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Average amount borrowed under the credit facility during the period.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityAverageOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The effective interest rate during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

Name:	us-gaap_LineOfCreditFacilityInterestRateDuringPeriod
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum amount borrowed under the credit facility at any time during the period.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumAmountOutstandingDuringPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_ShortTermDebtLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration