Investments - Scheduled maturities of available for sale fixed maturities (Details) $ in Millions	12 Months Ended
Dec. 31, 2020 USD ($)
Fair Value Percent, Fiscal Year Maturity [Abstract]
Average life of CLOs and Other ABS	3 years 6 months
Average life of MBS	3 years 6 months
Fixed maturities
Amortized Cost Basis, Fiscal Year Maturity [Abstract]
One year or less	$ 2,735
After one year through five years	10,344
After five years through ten years	9,298
After ten years	2,771
Fixed maturities amortized cost, Subtotal	25,148
Collateralized loan obligations and other ABS (average life of approximately 3-1/2 years)	11,663
MBS (average life of approximately 3-1/2 years)	3,557
Amortized Cost	40,368
Fair Value, Fiscal Year Maturity [Abstract]
One year or less	2,762
After one year through five years	11,149
After five years through ten years	10,570
After ten years	3,105
Fixed maturities fair value, Subtotal	27,586
Collateralized loan obligations and other ABS (average life of approximately 3-1/2 years)	11,776
MBS (average life of approximately 3-1/2 years)	3,845
Fair Value	$ 43,207
Fair Value Percent, Fiscal Year Maturity [Abstract]
One year or less	6.00%
After one year through five years	26.00%
After five years through ten years	25.00%
After ten years	7.00%
Fixed maturities fair value, Subtotal, Percent	64.00%
Collateralized loan obligations and other ABS (average life of approximately 3-1/2 years)	27.00%
MBS (average life of approximately 3-1/2 years)	9.00%
Fair value, Total, Percent	100.00%

Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Fair Value, Percent.

No definition available.

Name:	afg_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValuePercent
Namespace Prefix:	afg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Available-for-sale Securities, Debt Maturities, after One Through Five Years, Fair Value, Percent.

+ References

No definition available.

+ Details

Name:	afg_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValuePercent
Namespace Prefix:	afg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value, Percent.

+ References

No definition available.

+ Details

Name:	afg_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValuePercent
Namespace Prefix:	afg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Available for sale securities debt maturities excluding ABS and MBS amortized cost.

+ References

No definition available.

+ Details

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

- Definition

Available for sale securities debt maturities excluding ABS and MBS fair value.

+ References

No definition available.

+ Details

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

- Definition

Available for sale securities debt maturities excluding ABS and MBS fair value percent.

+ References

No definition available.

+ Details

Name:	afg_AvailableForSaleSecuritiesDebtMaturitiesExcludingAbsAndMbsFairValuePercent
Namespace Prefix:	afg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Available-for-sale Securities, Debt Maturities, Fair Value, Total, Percent.

+ References

No definition available.

+ Details

Name:	afg_AvailableForSaleSecuritiesDebtMaturitiesFairValuePercent
Namespace Prefix:	afg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Available For Sale Securities, Debt Maturities, Fair Value Percent, Fiscal Year Maturity [Abstract]

+ References

No definition available.

+ Details

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

- Definition

Available-for-sale Securities, Debt Maturities, within One Year, Fair Value, Percent.

+ References

No definition available.

+ Details

Name:	afg_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValuePercent
Namespace Prefix:	afg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Mortgage-backed securities, amortized cost.

+ References

No definition available.

+ Details

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

- Definition

Mortgage-backed securities, fair value, percent.

+ References

No definition available.

+ Details

Name:	afg_AvailableForSaleSecuritiesMbsFairValuePercent
Namespace Prefix:	afg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Collateralized loan obligations and Other asset-backed securities average useful life.

+ References

No definition available.

+ Details

Name:	afg_CollateralizedLoanObligationsandOtherAssetBackedSecuritiesAverageUsefulLife
Namespace Prefix:	afg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Collateralized loan obligations and Other asset-backed securities, amortized cost.

+ References

No definition available.

+ Details

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

- Definition

Collateralized loan obligations and other Asset-backed securities, fair value.

+ References

No definition available.

+ Details

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

- Definition

Collateralized loan obligations and other Asset-backed securities, fair value, percent.

+ References

No definition available.

+ Details

Name:	afg_CollateralizedLoanObligationsandOtherAvailableforSaleSecuritiesABSFairValuePercent
Namespace Prefix:	afg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Mortgage backed securities average useful life.

+ References

No definition available.

+ Details

Name:	afg_MortgageBackedSecuritiesAverageUsefulLife
Namespace Prefix:	afg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

No definition available.

+ Details

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

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

No definition available.

+ Details

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

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27198-111563

+ Details

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

- Definition

Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27198-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 3A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=120252992&loc=SL120269850-112803

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

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

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27198-111563

+ Details

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

- Definition

Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27198-111563

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 3A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120252992&loc=SL120269850-112803

+ Details

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

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing after tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27198-111563

+ Details

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

- Definition

Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing after tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 3A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=120252992&loc=SL120269850-112803

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27198-111563

+ Details

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

- References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27198-111563

+ Details

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

- Definition

Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 3A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120252992&loc=SL120269850-112803

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27198-111563

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

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

- Definition

Fair value of securitized, pay-through debt securities collateralized by real estate mortgage loans classified as available-for-sale.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_FixedMaturitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: