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

No definition available.

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

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No definition available.

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

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Fair value amount of debt instrument that was repurchased.

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No definition available.

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Face (par) amount of the original debt instrument that was repurchased.

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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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Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

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Amount of cash paid for interest. Includes, but is not limited to, payment to settle zero-coupon bond attributable to accreted interest of debt discount and debt instrument with insignificant coupon interest rate in relation to effective interest rate of borrowing attributable to accreted interest of debt discount.

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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 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 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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Interest received on loans and other debt instruments during the current period.

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Long-Term Debt - Narrative (Details) - USD ($)	1 Months Ended	12 Months Ended
Long-Term Debt - Narrative (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 11, 2017	Nov. 30, 2017	Aug. 31, 2017	Jun. 30, 2017	Nov. 10, 2016	Aug. 31, 2016	Nov. 30, 2015	Sep. 30, 2015	Jun. 30, 2015
Long-term Debt, Fiscal Year Maturity [Abstract]
Scheduled principal payments on debt in next year	$ 0	$ 0
Scheduled principal payments on debt in year two	0	0
Scheduled principal payments on debt in year three	0	0
Scheduled principal payments on debt in year four	0	0
Scheduled principal payments on debt in year five	0	0
Scheduled principal payments on debt after year five	1,320,000,000	1,320,000,000
Principal	1,318,000,000	1,318,000,000	$ 1,308,000,000
Cash interest payments on long-term debt		85,000,000	75,000,000	$ 75,000,000
AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Revolving credit line	500,000,000	500,000,000
Amount borrowed under AFG 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
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
Long-term Debt, Fiscal Year Maturity [Abstract]
Cash interest received on long-term debt affiliated with interest rate swap		$ 2,000,000	3,000,000	$ 2,000,000
Interest rate swaps \| Fair Value Hedge \| Designated as Hedging Instrument \| LIBOR \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Basis spread on variable interest rate					8.099%
Long-term debt \| Interest rate swaps \| Fair Value Hedge \| Designated as Hedging Instrument \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Net derivatives, at fair value			1,000,000
Senior Notes \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal	1,018,000,000	1,018,000,000	1,008,000,000
Senior Notes \| 4.50% Senior Notes due June 2047 \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Senior Notes, aggregate principal amount						$ 240,000,000		$ 350,000,000
Senior Notes, issuance price as a percentage of par value						102.374%		99.46%
Principal	$ 590,000,000	$ 590,000,000	0
Interest rate on debt instruments	4.50%	4.50%				4.50%		4.50%
Senior Notes \| 3.50% Senior Notes due August 2026 \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Senior Notes, aggregate principal amount						$ 125,000,000				$ 300,000,000
Senior Notes, issuance price as a percentage of par value						99.698%				99.608%
Principal	$ 425,000,000	$ 425,000,000	300,000,000
Interest rate on debt instruments	3.50%	3.50%				3.50%				3.50%
Senior Notes \| 9-7/8% Senior Notes due June 2019
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate on debt instruments	9.875%	9.875%											9.875%
Senior Notes \| 9-7/8% Senior Notes due June 2019 \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Senior Notes, amount of the original debt instrument that was redeemed					$ 350,000,000
Senior Notes, amount of the original debt instrument that was redeemed plus a make-whole premium					$ 388,000,000
Principal	$ 0	$ 0	$ 350,000,000
Interest rate on debt instruments	9.875%	9.875%	9.875%		9.875%								9.875%
Senior Notes \| 9-7/8% Senior Notes due June 2019 \| Interest rate swaps \| Fair Value Hedge \| Designated as Hedging Instrument \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Make-whole premium included in the loss on the retirement of debt	$ (1,000,000)
Senior Notes \| 6-3/8% Senior Notes due June 2042 \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Senior Notes, amount of the original debt instrument that was redeemed								$ 230,000,000
Principal	$ 0	$ 0	$ 230,000,000
Interest rate on debt instruments	6.375%	6.375%						6.375%
Senior Notes \| 5-3/4% Senior Notes due August 2042 \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Senior Notes, amount of the original debt instrument that was redeemed							$ 125,000,000
Principal	$ 0	$ 0	125,000,000
Interest rate on debt instruments	5.75%	5.75%					5.75%
Senior Notes \| 7% Senior Notes due September 2050
Long-term Debt, Fiscal Year Maturity [Abstract]
Senior Notes, amount of the original debt instrument that was redeemed												$ 132,000,000
Senior Notes \| 7% Senior Notes due September 2050 \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate on debt instruments												7.00%
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
Interest rate on debt instruments	6.25%	6.25%
Subordinated Debt \| 6% Subordinated Debentures due November 2055 \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Senior Notes, aggregate principal amount											$ 150,000,000
Principal	$ 150,000,000	$ 150,000,000	$ 150,000,000
Interest rate on debt instruments	6.00%	6.00%									6.00%
Revolving Credit Facility \| National Interstate
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal									$ 18,000,000
Revolving Credit Facility \| Line of Credit \| National Interstate
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal									$ 6,000,000
Make-whole premium \| Senior Notes \| 9-7/8% Senior Notes due June 2019 \| AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Make-whole premium included in the loss on the retirement of debt	$ (38,000,000)