Income Taxes - Narrative (Details) £ in Millions, $ in Millions			1 Months Ended	3 Months Ended			12 Months Ended
		Nov. 10, 2016 USD ($)	Jan. 31, 2008 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2016 GBP (£)
Income Tax Reconciliation [Line Items]
Statutory rate of income taxes							35.00%	35.00%	35.00%
Income tax expense (benefit)							$ 119	$ 195	$ 220
Payments to acquire additional interest in subsidiaries							$ 315	0	0
Effective tax rate excluding the charge related to the exit of certain lines of business within subsidiaries							35.00%
Income (losses) subject to tax in foreign jurisdictions							$ (160)	(66)	(4)
Operating loss carryforwards, valuation allowance				$ 50			$ 50
Percentage ownership of National Interstate needed to be included in AFG's consolidated tax group				80.00%			80.00%				80.00%
Decrease in AFG's liability for uncertain tax positions						$ (19)
Decrease in AFG's liability for uncertain tax positions, settlements						(11)	$ 0	0	(11)
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 prior year							0	0	(8)
Income tax penalties accrued								0	0
Cash payments for income taxes							308	234	347
National Interstate
Income Tax Reconciliation [Line Items]
Income tax expense (benefit)				$ (66)
Release of deferred tax liabilities		$ 66
Neon Capital Limited
Income Tax Reconciliation [Line Items]
Income tax expense (benefit)				(111)
Increase (Decrease) in current taxes payable				(29)
Charge for valuation allowance
Income Tax Reconciliation [Line Items]
Charge for valuation allowance against deferred tax assets							120
AFG
Income Tax Reconciliation [Line Items]
Income tax expense (benefit)				(177)			119	195	220
Interest included in tax provision									(1)
Decrease in related accrued interest (are less than in 2016 and 2015)									0
Amount of expense for penalties related to a tax position								0	$ 0
Maximum | AFG
Income Tax Reconciliation [Line Items]
Interest included in tax provision							1	1
Decrease in related accrued interest (are less than in 2016 and 2015)				1			1	1
Amount of expense for penalties related to a tax position							1
Income tax penalties accrued				1			1
Timing of recognition of investment income
Income Tax Reconciliation [Line Items]
Decrease in AFG's liability for uncertain tax positions						(17)
Deductibility of certain financing expenses
Income Tax Reconciliation [Line Items]
Decrease in AFG's liability for uncertain tax positions						$ (2)
United Kingdom
Income Tax Reconciliation [Line Items]
Operating loss carryforwards	[1]			$ 139			139				£ 112
Property and casualty insurance
Income Tax Reconciliation [Line Items]
Change in AFG’s liability for uncertain tax positions due to the uncertainty in state taxation								$ 1
Property and casualty insurance | Other lines
Income Tax Reconciliation [Line Items]
Special charges related to the exit of certain lines of business within subsidiaries					$ 65
Property and casualty insurance | Other lines | Neon Capital Limited
Income Tax Reconciliation [Line Items]
Special charges related to the exit of certain lines of business within subsidiaries							$ 65
Neon Capital Limited
Income Tax Reconciliation [Line Items]
Payments to acquire controlling interest in subsidiaries			$ 75
Percentage ownership of subsidiary			67.00%
Payments to acquire additional interest in subsidiaries										$ 17

[1] £112 million