Fair Value Measurements - Unobservable inputs used in determining fair value of embedded derivatives (Details) - Embedded derivatives	9 Months Ended
Sep. 30, 2018
Unobservable inputs used by management in determining fair value of embedded derivatives
Risk margin for uncertainty in cash flows	0.70%
Minimum
Unobservable inputs used by management in determining fair value of embedded derivatives
Adjustment for insurance subsidiary’s credit risk	0.40%
Surrenders	3.00%
Partial surrenders	2.00%
Annuitizations	0.10%
Deaths	1.60%
Budgeted option costs	2.40%
Maximum
Unobservable inputs used by management in determining fair value of embedded derivatives
Adjustment for insurance subsidiary’s credit risk	1.60%
Surrenders	23.00%
Partial surrenders	9.00%
Annuitizations	1.00%
Deaths	8.00%
Budgeted option costs	3.60%

- Definition

Risk associated with risk of adjustment of an insurance subsidiary’s credit risk, expressed as a percentage, used as an input to measure fair value.

+ References

No definition available.

+ Details

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

- Definition

Risk associated with risk of annuitizations, expressed as a percentage, used as an input to measure fair value.

+ References

No definition available.

+ Details

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

- Definition

Risk associated with risk of budgeted option costs, expressed as a percentage, used as an input to measure fair value.

+ References

No definition available.

+ Details

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

- Definition

Risk associated with risk of deaths, expressed as a percentage, used as an input to measure fair value.

+ References

No definition available.

+ Details

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

- Definition

Risk associated with risk of partial surrenders, expressed as a percentage, used as an input to measure fair value.

+ References

No definition available.

+ Details

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

- Definition

Risk associated with risk margin for uncertainty in cash flows, expressed as a percentage, used as an input to measure fair value.

+ References

No definition available.

+ Details

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

- Definition

Risk associated with risk of surrenders, expressed as a percentage, used as an input to measure fair value.

+ References

No definition available.

+ Details

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

- Definition

Unobservable inputs used in determining fair value of embedded derivatives.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_EmbeddedDerivativeFinancialInstrumentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: