Income Taxes (Details Textual) £ in Millions, $ in Millions		3 Months Ended	12 Months Ended
		Sep. 30, 2014 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2015 GBP (£)
Income Tax Reconciliation [Line Items]
Statutory rate of income taxes			35.00%	35.00%	35.00%
Income (losses) subject to tax in foreign jurisdictions			$ (66)	$ (4)	$ 4
Percentage ownership of National Interstate			51.00%			51.00%
Operating loss carryforwards, valuation allowance			$ 50
Decrease in AFG's liability for uncertain tax positions		$ (19)
Decrease in AFG's liability for uncertain tax positions, settlements		(11)	0	(11)	0
The portion of the reduction in the liability for uncertain tax positions that favorably impacted the effective tax rate		4
Change in AFG's liability for uncertain tax positions of current year, uncertainty as to timing of tax return inclusion of income related to certain securities					1
Change in AFG's liability for uncertain tax positions of prior year			0	(8)	0
Income tax penalties accrued			0		0
Cash payments for income taxes			234	347	204
AFG [Member]
Income Tax Reconciliation [Line Items]
Interest included in tax provision				$ (1)	1
Decrease in related accrued interest					$ 1
Charge for valuation allowance [Member]
Income Tax Reconciliation [Line Items]
Charge for valuation allowance against deferred tax assets			77
Maximum [Member] | AFG [Member]
Income Tax Reconciliation [Line Items]
Interest included in tax provision			1
Decrease in related accrued interest			1
United Kingdom [Member]
Income Tax Reconciliation [Line Items]
Operating loss carryforwards	[1]		133			£ 90
Timing of recognition of investment income [Member]
Income Tax Reconciliation [Line Items]
Decrease in AFG's liability for uncertain tax positions		(17)
Deductibility of certain financing expenses [Member]
Income Tax Reconciliation [Line Items]
Decrease in AFG's liability for uncertain tax positions		$ (2)
Property and Casualty Insurance [Member]
Income Tax Reconciliation [Line Items]
Change in AFG’s liability for uncertain tax positions of the current year due to the uncertainty in state taxation			$ 1

[1] £90 million