Managed Investment Entities (Details Textual) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2012 CollateralizedLoanObligations	Dec. 31, 2011
Managed Investment Entities (Textual) [Abstract]
Percentage of investment of most subordinate debt tranche, Minimum	7.50%
Percentage of investment of most subordinate debt tranche, Maximum	51.20%
Number of collateralized loan obligation entities	8
Difference between aggregate unpaid principal balance and fair value of CLOs' fixed maturity investments	$ 65	$ 120
Difference between aggregate unpaid principal balance and fair value of CLOs' debt	227	306
Aggregate unpaid principal balance of CLO loans in default	17	7
Carrying amount of CLO loans in default	6	2
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Aggregate fair value of investment in collateralized loan obligations	135	98
New CLO Formed in March 2012 [Member]
Variable Interest Entity [Line Items]
Face amount of liabilities	410
Face amount of liabilities purchased by subsidiaries of AFG	$ 39

Aggregate unpaid principal balance of variable interest entity loans in default.

No definition available.

Name:	afg_AggregateUnpaidPrincipalBalanceOfVariableInterestEntityLoansInDefault
Namespace Prefix:	afg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Carrying amount of variable interest entity loans in default.

+ References

No definition available.

+ Details

Name:	afg_CarryingAmountOfVariableInterestEntityLoansInDefault
Namespace Prefix:	afg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Debt instrument face amount of liabilities purchased by subsidiaries.

+ References

No definition available.

+ Details

Name:	afg_DebtInstrumentFaceAmountOfLiabilitiesPurchasedBySubsidiaries
Namespace Prefix:	afg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Difference between aggregate unpaid principal balance and fair value of variable interest entity debt.

+ References

No definition available.

+ Details

Name:	afg_DifferenceBetweenAggregateUnpaidPrincipalBalanceAndFairValueOfVariableInterestEntityDebt
Namespace Prefix:	afg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Difference between aggregate unpaid principal balance and fair value of variable interest entity investments.

+ References

No definition available.

+ Details

Name:	afg_DifferenceBetweenAggregateUnpaidPrincipalBalanceAndFairValueOfVariableInterestEntityInvestments
Namespace Prefix:	afg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Face value of liabilities issued by variable interest entity.

+ References

No definition available.

+ Details

Name:	afg_FaceValueOfLiabilitiesIssuedByVariableInterestEntity
Namespace Prefix:	afg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	afg_ManagedInvestmentEntitiesTextualAbstract
Namespace Prefix:	afg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of collateralized loan obligation entities.

+ References

No definition available.

+ Details

Name:	afg_NumberOfCollateralizedLoanObligationEntities
Namespace Prefix:	afg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum percentage of investment of most subordinate debt tranche of variable interest entity.

+ References

No definition available.

+ Details

Name:	afg_PercentageOfInvestmentOfMostSubordinateDebtTrancheOfVariableInterestEntityMaximum
Namespace Prefix:	afg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Minimum percentage of investment of most subordinate debt tranche of variable interest entity.

+ References

No definition available.

+ Details

Name:	afg_PercentageOfInvestmentOfMostSubordinateDebtTrancheOfVariableInterestEntityMinimum
Namespace Prefix:	afg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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 investment securities as of the balance sheet date which may include marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Available-for-Sale Securities

-URI http://asc.fasb.org/extlink&oid=6505594

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6379932&loc=d3e22054-111558

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 7

-Section 03

-Paragraph 1

-Subparagraph a, f, g

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 35

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6956420&loc=d3e24584-111560

+ Details

Name:	us-gaap_InvestmentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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