Long-Term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Summary of Carrying value of long-term debt
Debt issue costs	$ 22	$ 22
Long-term debt	998	998
Including Deferred Debt Issue Costs [Member]
Summary of Carrying value of long-term debt
Long-term debt	1,020	1,020
Subsidiaries [Member]
Summary of Carrying value of long-term debt
Debt issue costs	0	0
National Interstate bank credit facility	12	12
Subsidiaries [Member] \| Including Deferred Debt Issue Costs [Member]
Summary of Carrying value of long-term debt
National Interstate bank credit facility	12	12
Senior Notes [Member] \| 9-7/8% Senior Notes due June 2019 [Member] \| AFG [Member]
Summary of Carrying value of long-term debt
Senior Notes	349	349
Debt issue costs	1	1
Senior Notes [Member] \| 9-7/8% Senior Notes due June 2019 [Member] \| AFG [Member] \| Including Deferred Debt Issue Costs [Member]
Summary of Carrying value of long-term debt
Senior Notes	350	350
Senior Notes [Member] \| 6-3/8% Senior Notes due June 2042 [Member] \| AFG [Member]
Summary of Carrying value of long-term debt
Senior Notes	223	223
Debt issue costs	7	7
Senior Notes [Member] \| 6-3/8% Senior Notes due June 2042 [Member] \| AFG [Member] \| Including Deferred Debt Issue Costs [Member]
Summary of Carrying value of long-term debt
Senior Notes	230	230
Senior Notes [Member] \| 5-3/4% Senior Notes due August 2042 [Member] \| AFG [Member]
Summary of Carrying value of long-term debt
Senior Notes	121	121
Debt issue costs	4	4
Senior Notes [Member] \| 5-3/4% Senior Notes due August 2042 [Member] \| AFG [Member] \| Including Deferred Debt Issue Costs [Member]
Summary of Carrying value of long-term debt
Senior Notes	125	125
Senior Debt Obligations [Member] \| AFG [Member]
Summary of Carrying value of long-term debt
Debt issue costs	12	12
Long-term debt	696	696
Senior Debt Obligations [Member] \| AFG [Member] \| Including Deferred Debt Issue Costs [Member]
Summary of Carrying value of long-term debt
Long-term debt	708	708
Senior Debt Obligations [Member] \| Other [Member] \| AFG [Member]
Summary of Carrying value of long-term debt
Other long-term debt	3	3
Debt issue costs	0	0
Senior Debt Obligations [Member] \| Other [Member] \| AFG [Member] \| Including Deferred Debt Issue Costs [Member]
Summary of Carrying value of long-term debt
Other long-term debt	3	3
Subordinated Debentures [Member] \| 6-1/4% Subordinated Debentures due September 2054 [Member] \| AFG [Member]
Summary of Carrying value of long-term debt
Debt issue costs	5	5
Subordinated Debentures	145	145
Subordinated Debentures [Member] \| 6-1/4% Subordinated Debentures due September 2054 [Member] \| AFG [Member] \| Including Deferred Debt Issue Costs [Member]
Summary of Carrying value of long-term debt
Subordinated Debentures	150	150
Subordinated Debentures [Member] \| 6% Subordinated Debentures due November 2055 [Member] \| AFG [Member]
Summary of Carrying value of long-term debt
Debt issue costs	5	5
Subordinated Debentures	145	145
Subordinated Debentures [Member] \| 6% Subordinated Debentures due November 2055 [Member] \| AFG [Member] \| Including Deferred Debt Issue Costs [Member]
Summary of Carrying value of long-term debt
Subordinated Debentures	150	150
Subordinated Debt Obligations [Member] \| AFG [Member]
Summary of Carrying value of long-term debt
Debt issue costs	10	10
Long-term debt	290	290
Subordinated Debt Obligations [Member] \| AFG [Member] \| Including Deferred Debt Issue Costs [Member]
Summary of Carrying value of long-term debt
Long-term debt	$ 300	$ 300

Summary of Carrying value of long-term debt.

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For an unclassified balance sheet, the carrying amount (net of accumulated amortization) as of the balance sheet date of capitalized costs associated with the issuance of debt instruments (for example, legal, accounting, underwriting, printing, and registration costs) that will be charged against earnings over the life of the debt instruments to which such costs pertain.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of loans from a bank with maturities initially due after one year or beyond the normal operating cycle if longer.

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Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Including both current and noncurrent portions, carrying amount as of the balance sheet date of other forms of debt not elsewhere specified in the taxonomy with initial maturities beyond one year or beyond the normal operating cycle, if longer.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of subordinated debt (with initial maturities beyond one year or beyond the operating cycle if longer). Subordinated debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets.

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