Insurance - Recoverables from reinsurance, progression of allowance for expected credit losses (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Jan. 01, 2020
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 18
Impact of adoption of new accounting policy	13	$ 6
Provision for expected credit losses	2
Write-offs charged against the allowance	0
Businesses disposed	(1)
Ending balance	13
Property and casualty insurance
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning balance	18
Impact of adoption of new accounting policy	6	11
Provision for expected credit losses	0
Write-offs charged against the allowance	0
Businesses disposed	(1)
Ending balance	6
Annuity
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0
Impact of adoption of new accounting policy	0	0
Provision for expected credit losses	0
Write-offs charged against the allowance	0
Businesses disposed	0
Ending balance	0
Other
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0
Impact of adoption of new accounting policy	7	$ (5)
Provision for expected credit losses	2
Write-offs charged against the allowance	0
Businesses disposed	0
Ending balance	$ 7

Reinsurance recoverable, allowance for credit loss, businesses disposed

No definition available.

Name:	afg_ReinsuranceRecoverableAllowanceForCreditLossBusinessesDisposed
Namespace Prefix:	afg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of the write-down or write-off of a receivable from a reinsurer.

+ References

+ Details

Name:	us-gaap_ReinsuranceLossOnUncollectibleAccountsInPeriodAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount of expense (reversal of expense) for expected credit loss on reinsurance recoverable.

+ References

+ Details

Name:	us-gaap_ReinsuranceRecoverableCreditLossExpenseReversal
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of allowance for credit loss on reinsurance recoverable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121599878&loc=SL82895884-210446

+ Details

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

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=afg_PropertyAndCasualtyInsuranceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=afg_AnnuityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=us-gaap_CorporateAndOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: