Income Taxes - Narrative (Details) £ in Millions, $ in Millions			1 Months Ended	3 Months Ended		12 Months Ended
		Jan. 01, 2018	Jan. 31, 2008 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2018 GBP (£)
Income Tax Reconciliation [Line Items]
Statutory rate of income taxes						21.00%	35.00%	35.00%
Payments to acquire additional interest in subsidiaries						$ 0	$ 0	$ 315
Impact of foreign operations included in the provision for income taxes						(2)	21	5
Decrease in the valuation allowance related to the change in the U.S. corporate tax rate						11	(7)	52
Income tax expense (benefit)						122	247	119
Decrease in net deferred tax assets resulting from the changes in the Tax Cuts and Jobs Act of 2017						0	$ 83	$ 0
Effective tax rate excluding the charge related to the exit of certain lines of business within subsidiaries and impact of the change in the U.S. corporate tax rate							31.00%
Effective tax rate excluding the charge related to the exit of certain lines of business within subsidiaries, acquisition of the noncontrolling interest, and restructuring of subsidiaries								35.00%
Income (losses) subject to tax in foreign jurisdictions						(69)	$ (58)	$ (160)
Operating loss carryforwards, valuation allowance						23
Cash payments for income taxes						$ 156	194	308
Limitation on the use of tax credit carryforwards						A SRLY NOL can be used only by the entity that created it and only in years that both it and the consolidated group have taxable income. Approximately $19 million of AFG’s SRLY NOLs expired unutilized at December 31, 2018.
Tax Cuts and Jobs Act implementation complete						true
AFG
Income Tax Reconciliation [Line Items]
Income tax expense (benefit)				$ (56)		$ 122	247	119
National Interstate
Income Tax Reconciliation [Line Items]
Income tax expense (benefit)					$ (66)
Neon Capital Limited
Income Tax Reconciliation [Line Items]
Income tax expense (benefit)				(56)	(111)			(48)
Increase (Decrease) in current taxes payable				(20)	$ (29)
Charge for valuation allowance
Income Tax Reconciliation [Line Items]
Charge for valuation allowance against deferred tax assets						93
Maximum | AFG
Income Tax Reconciliation [Line Items]
Unrecognized tax benefits that would impact the effective tax rate				1		1	1
Interest included in tax provision						1	1	1
Decrease in related accrued interest (are less than in 2017 and 2016)				1			1
Amount of expense for penalties related to a tax position						1	1	1
Income tax penalties accrued				1		1	1
Amount unutilized
Income Tax Reconciliation [Line Items]
Operating loss carryforwards						19
Tax Cuts and Jobs Act of 2017
Income Tax Reconciliation [Line Items]
Statutory rate of income taxes		21.00%
Decrease in net deferred tax assets resulting from the changes in the Tax Cuts and Jobs Act of 2017				$ 83
Tax Cuts and Jobs Act of 2017 | Neon Capital Limited
Income Tax Reconciliation [Line Items]
Decrease in the valuation allowance related to the change in the U.S. corporate tax rate							(61)
United Kingdom
Income Tax Reconciliation [Line Items]
Operating loss carryforwards	[1]					$ 215					£ 168
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
Tax payments due to resolved 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
Foreign underwriting losses
Income Tax Reconciliation [Line Items]
Impact of foreign operations included in the provision for income taxes							$ 14

[1] £168 million