Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Contractual interest rate for funds borrowed, under the debt agreement.

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The amount of cash paid for interest during the period.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Description of the interest rate for the amounts borrowed under the credit facility, including the terms and the method for determining the interest rate (for example, fixed or variable, LIBOR plus a percentage, increasing rate, timing of interest rate resets, remarketing provisions).

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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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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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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The cash inflow from a borrowing with the highest claim on the assets of the entity in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the 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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Long-Term Debt (Details Textual) (USD $) In Millions, unless otherwise specified	12 Months Ended			0 Months Ended	1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Jun. 30, 2012	Aug. 31, 2012	Sep. 30, 2014
Debt Instrument [Line Items]
Scheduled principal payments on debt in next year	$ 14			$ 14
Scheduled principal payments on debt in year two	45			45
Scheduled principal payments on debt in year three	12			12
Scheduled principal payments on debt in year four	0			0
Scheduled principal payments on debt in year five	350			350
Scheduled principal payments on debt after year five	640			640
Cash interest payments on long-term debt	72us-gaap_InterestPaid	71us-gaap_InterestPaid	75us-gaap_InterestPaid
AFG [Member]
Debt Instrument [Line Items]
Revolving credit line	500			500
Interest rate on revolving debt facility				1.375%
Amount borrowed under AFG revolving credit facility	0	0		0
Interest rate description for revolving credit facility	1.00% to 1.875% (currently 1.375%) over LIBOR
AFG [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Interest rate on revolving debt facility				1.00%
AFG [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Interest rate on revolving debt facility				1.875%
National Interstate [Member]
Debt Instrument [Line Items]
Revolving credit line	100			100
Interest rate on revolving debt facility				1.20%
Loans payable under National Interstate bank credit facility	12			12
Interest rate description for revolving credit facility	six-month LIBOR plus 0.875% (currently 1.20%)
National Interstate [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Interest rate on revolving debt facility				0.875%
9-7/8% Senior Notes due June 2019 [Member]
Debt Instrument [Line Items]
Interest rate on debt instruments	9.875%			9.875%
9-7/8% Senior Notes due June 2019 [Member] \| AFG [Member]
Debt Instrument [Line Items]
Senior Notes	350	350		350
6-3/8% Senior Notes due June 2042 [Member]
Debt Instrument [Line Items]
Interest rate on debt instruments	6.375%			6.375%
6-3/8% Senior Notes due June 2042 [Member] \| AFG [Member]
Debt Instrument [Line Items]
Senior Notes	230	230		230
Proceeds from Issuance of Senior Long-term Debt					230
5-3/4% Senior Notes due August 2042 [Member]
Debt Instrument [Line Items]
Interest rate on debt instruments	5.75%			5.75%
5-3/4% Senior Notes due August 2042 [Member] \| AFG [Member]
Debt Instrument [Line Items]
Senior Notes	125	125		125
Proceeds from Issuance of Senior Long-term Debt						125
7% Senior Notes due September 2050 [Member]
Debt Instrument [Line Items]
Interest rate on debt instruments	7.00%			7.00%
7% Senior Notes due September 2050 [Member] \| AFG [Member]
Debt Instrument [Line Items]
Senior Notes	132	132		132
6-1/4% Subordinated Debentures due September 2054 [Member]
Debt Instrument [Line Items]
Interest rate on debt instruments	6.25%			6.25%
6-1/4% Subordinated Debentures due September 2054 [Member] \| AFG [Member]
Debt Instrument [Line Items]
Subordinated Debentures	$ 150	$ 0		$ 150			$ 150
7-1/2% Senior Debentures due November 2033 [Member]
Debt Instrument [Line Items]
Interest rate on debt instruments					7.50%
7-1/4% Senior Debentures due January 2034 [Member]
Debt Instrument [Line Items]
Interest rate on debt instruments					7.25%
7-1/8% Senior Debentures due February 2034 [Member]
Debt Instrument [Line Items]
Interest rate on debt instruments						7.125%