Accounting Policies - Narrative on Revenue Recognition Guidance (Details)	6 Months Ended
Jun. 30, 2017
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Expected effect on consolidated revenues under new revenue recognition guidance effective in 2018	2.00%

Represents the percentage in which the effect of adopting the new accounting standard or change in accounting principle expected by the entity to have a significant effect on the entity's financial statements.

No definition available.

Name:	afg_NewAccountingPronouncementOrChangeInAccountingPrincipleEffectOfAdoptionOnRevenuesPercentage
Namespace Prefix:	afg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration