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Shareholders' Equity (Details) (USD $)
Mar. 31, 2015
Voting Preferred Stock [Member]  
Class of Stock [Line Items]  
Preferred Stock, par value $ 0us-gaap_PreferredStockNoParValue
/ us-gaap_StatementClassOfStockAxis
= afg_VotingPreferredStockMember
Preferred Stock, shares authorized 12,500,000.0us-gaap_PreferredStockSharesAuthorized
/ us-gaap_StatementClassOfStockAxis
= afg_VotingPreferredStockMember
Nonvoting Preferred Stock [Member]  
Class of Stock [Line Items]  
Preferred Stock, par value $ 0us-gaap_PreferredStockNoParValue
/ us-gaap_StatementClassOfStockAxis
= afg_NonvotingPreferredStockMember
Preferred Stock, shares authorized 12,500,000.0us-gaap_PreferredStockSharesAuthorized
/ us-gaap_StatementClassOfStockAxis
= afg_NonvotingPreferredStockMember