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Long-Term Debt (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Sep. 30, 2014
Dec. 31, 2013
Summary of Carrying value of long-term debt      
Long-term debt $ 1,061us-gaap_LongTermDebt   $ 913us-gaap_LongTermDebt
AFG [Member]      
Summary of Carrying value of long-term debt      
Long-term debt 990us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
  840us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Subsidiaries [Member]      
Summary of Carrying value of long-term debt      
Long-term debt 71us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
  73us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
Notes payable secured by real estate due 2015 through 2016 59afg_LongTermNotesPayableSecuredByRealEstate
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
  61afg_LongTermNotesPayableSecuredByRealEstate
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
National Interstate bank credit facility 12us-gaap_LoansPayableToBank
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
  12us-gaap_LoansPayableToBank
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
9-7/8% Senior Notes due June 2019 [Member] | AFG [Member]      
Summary of Carrying value of long-term debt      
Senior Notes 350us-gaap_SeniorNotes
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= afg_SeniorNotesDueInJune2019Member
  350us-gaap_SeniorNotes
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= afg_SeniorNotesDueInJune2019Member
6-3/8% Senior Notes due June 2042 [Member] | AFG [Member]      
Summary of Carrying value of long-term debt      
Senior Notes 230us-gaap_SeniorNotes
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= afg_SeniorNotesDueInJune2042Member
  230us-gaap_SeniorNotes
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= afg_SeniorNotesDueInJune2042Member
5-3/4% Senior Notes due August 2042 [Member] | AFG [Member]      
Summary of Carrying value of long-term debt      
Senior Notes 125us-gaap_SeniorNotes
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= afg_SeniorNotesDueInAugust2042Member
  125us-gaap_SeniorNotes
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= afg_SeniorNotesDueInAugust2042Member
7% Senior Notes due September 2050 [Member] | AFG [Member]      
Summary of Carrying value of long-term debt      
Senior Notes 132us-gaap_SeniorNotes
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= afg_SeniorNotesDueInSeptember2050Member
  132us-gaap_SeniorNotes
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= afg_SeniorNotesDueInSeptember2050Member
Senior Debt Obligations [Member] | AFG [Member]      
Summary of Carrying value of long-term debt      
Other long-term debt 3us-gaap_OtherLongTermDebt
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorDebtObligationsMember
  3us-gaap_OtherLongTermDebt
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorDebtObligationsMember
Long-term debt 840us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorDebtObligationsMember
  840us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorDebtObligationsMember
6-1/4% Subordinated Debentures due September 2054 [Member] | AFG [Member]      
Summary of Carrying value of long-term debt      
Subordinated Debentures $ 150us-gaap_SubordinatedDebt
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= afg_SubordinatedDebenturesDueInSeptember2054Member
$ 150us-gaap_SubordinatedDebt
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= afg_SubordinatedDebenturesDueInSeptember2054Member
$ 0us-gaap_SubordinatedDebt
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= afg_SubordinatedDebenturesDueInSeptember2054Member