Represents the debt instrument’s issuance price as a percentage of the debt instrument's face amount.

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Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing after the sixth fiscal year following the latest fiscal year.

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Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the sixth fiscal year following the latest fiscal year ended.

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

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Face (par) amount of debt instrument at time of issuance.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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No definition available.

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The percentage points added to the reference rate to compute the variable rate on the interest rate derivative.

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No definition available.

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Fair value of the assets less the liabilities of a derivative or group of derivatives.

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Contains all pertinent information about an entity's fair value hedging activities.

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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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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 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 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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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 remainder of the 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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Long-Term Debt - Narrative (Details) - USD ($)		1 Months Ended	9 Months Ended
Long-Term Debt - Narrative (Details) - USD ($)	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2016	Aug. 31, 2016	Dec. 31, 2015
Long-term Debt, Fiscal Year Maturity [Abstract]
Scheduled principal payments on debt in 2016	$ 18,000,000		$ 18,000,000
Scheduled principal payments on debt in 2017	0		0
Scheduled principal payments on debt in 2018	0		0
Scheduled principal payments on debt in 2019	350,000,000		350,000,000
Scheduled principal payments on debt in 2020	0		0
Scheduled principal payments on debt in 2021	0		0
Scheduled principal payments on debt after 2021	958,000,000		958,000,000
Principal	1,326,000,000		1,326,000,000		$ 1,020,000,000
AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Revolving credit line term		5 years
Revolving credit line	500,000,000		500,000,000
Amount borrowed under revolving credit facility	$ 0		$ 0		$ 0
Interest rate description of fair value hedge activity			three-month LIBOR plus 8.099%
Interest rate description for revolving credit facility			1.00% to 1.875% (currently 1.375%) over LIBOR
National Interstate
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate description for revolving credit facility			LIBOR plus 0.875% (currently 1.518%)
LIBOR \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate on revolving debt facility	1.375%
LIBOR \| AFG \| Minimum
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate on revolving debt facility	1.00%
LIBOR \| AFG \| Maximum
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate on revolving debt facility	1.875%
Interest rate swaps \| Fair Value Hedge \| Designated as Hedging Instrument \| AFG
Debt Instrument [Line Items]
Effective interest rate, percentage	8.9493%		8.9493%		8.611%
Interest rate swaps \| Fair Value Hedge \| Designated as Hedging Instrument \| LIBOR \| AFG
Debt Instrument [Line Items]
Basis spread on variable interest rate	8.099%		8.099%
Long-term debt \| Interest rate swaps \| Fair Value Hedge \| Designated as Hedging Instrument \| AFG
Debt Instrument [Line Items]
Net derivatives, at fair value	$ 6,000,000		$ 6,000,000		$ 2,000,000
Senior Notes \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal	$ 1,008,000,000		$ 1,008,000,000		708,000,000
Senior Notes \| 9-7/8% Senior Notes due June 2019
Long-term Debt, Fiscal Year Maturity [Abstract]
Stated interest rate, percentage	9.875%		9.875%
Senior Notes \| 9-7/8% Senior Notes due June 2019 \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal	$ 350,000,000		$ 350,000,000		$ 350,000,000
Stated interest rate, percentage	9.875%		9.875%		9.875%
Senior Notes \| 3-1/2% Senior Notes due August 2026 \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Senior Notes, aggregate principal amount				$ 300,000,000
Senior Notes, issuance price as a percentage of par value				99.608%
Principal	$ 300,000,000		$ 300,000,000		$ 0
Stated interest rate, percentage	3.50%		3.50%	3.50%
Senior Notes \| 6-3/8% Senior Notes due June 2042 \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal	$ 230,000,000		$ 230,000,000		230,000,000
Stated interest rate, percentage	6.375%		6.375%
Senior Notes \| 5-3/4% Senior Notes due August 2042 \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal	$ 125,000,000		$ 125,000,000		125,000,000
Stated interest rate, percentage	5.75%		5.75%
Subordinated Debt \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal	$ 300,000,000		$ 300,000,000		300,000,000
Subordinated Debt \| 6-1/4% Subordinated Debentures due September 2054 \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal	$ 150,000,000		$ 150,000,000		150,000,000
Stated interest rate, percentage	6.25%		6.25%
Subordinated Debt \| 6% Subordinated Debentures due November 2055 \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal	$ 150,000,000		$ 150,000,000		150,000,000
Stated interest rate, percentage	6.00%		6.00%
Revolving Credit Facility \| National Interstate
Long-term Debt, Fiscal Year Maturity [Abstract]
Revolving credit line	$ 100,000,000		$ 100,000,000
Principal	18,000,000		18,000,000		$ 12,000,000
Revolving Credit Facility \| Line of Credit \| National Interstate
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal	$ 6,000,000		$ 6,000,000