Derivatives (Details) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	$ 284	$ 499
Derivative liability, at fair value	1,207	1,173
MBS with embedded derivatives [Member] \| Fixed maturities [Member]
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	162	158
Public company warrants [Member] \| Equity securities [Member]
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	0	19
Fixed-indexed annuities (embedded derivative) [Member] \| Annuity benefits accumulated [Member]
Derivatives included in AFG' s Balance Sheet at fair value
Derivative liability, at fair value	1,198	1,160
Equity index call options [Member] \| Other investments [Member]
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	122	322
Reinsurance contracts (embedded derivative) [Member] \| Other liabilities [Member]
Derivatives included in AFG' s Balance Sheet at fair value
Derivative liability, at fair value	$ 9	$ 13

Fair value, after offset of derivative asset, of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 8

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=56946850&loc=d3e41678-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 50

-Paragraph 3

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 55

-Paragraph 9

-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226006-175313

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 55

-Paragraph 10

-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226008-175313

Name:	us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant