Income Taxes (Details Textual) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended			3 Months Ended		12 Months Ended									3 Months Ended
	Sep. 30, 2014 USD ($)	Sep. 30, 2012 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2012 Tax litigation settlement [Member] USD ($)	Sep. 30, 2012 Tax litigation settlement [Member] USD ($)	Dec. 31, 2014 AFG [Member] USD ($)	Dec. 31, 2013 AFG [Member] USD ($)	Dec. 31, 2012 AFG [Member] USD ($)	Dec. 31, 2014 Charge for valuation allowance [Member] USD ($)	Dec. 31, 2014 Maximum [Member] USD ($)	Dec. 31, 2014 United Kingdom [Member] USD ($)		Dec. 31, 2014 United Kingdom [Member] GBP (£)		Sep. 30, 2014 Timing of recognition of investment income [Member] USD ($)	Sep. 30, 2014 Deductibility of certain financing expenses [Member] USD ($)
Income Tax Reconciliation [Line Items]
Statutory rate of income taxes			35.00%	35.00%	35.00%
Tax benefit related to settlement of litigation						$ 39	$ 28
Expected refund of tax and interest paid related to tax settlements							17
Income (losses) subject to tax in foreign jurisdictions (are less than in 2014)				12	(3)							(1)
Operating loss carryforwards													131	[1]	85	[1]
Percentage ownership of National Interstate			51.00%
Charge for valuation allowance against deferred tax assets											56
Operating loss carryforwards, valuation allowance			50
Decrease in AFG's liability for uncertain tax positions	(19)																(17)	(2)
Decrease in AFG's liability for uncertain tax positions, settlements	(11)		(11)	0	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	3
Change in AFG's liability for uncertain tax positions of current year related to deductibility of certain expenses					2
Change in AFG's liability for uncertain tax positions of prior year related to resolution of tax case					36
Change in AFG's related liability for interest for uncertain tax positions					14
Change in AFG's liability for uncertain tax positions of prior year		(10)	(8)	0	(46)
Interest included in tax provision								(1)	1	(14)
Decrease in related accrued interest				1
Income tax penalties accrued				0
Cash payments for income taxes			$ 347	$ 204	$ 277

[1] £85 million