Income Taxes - Reconciliation of income taxes at the statutory rate to the provision for income taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Earnings before income taxes (“EBT”)	$ 29	$ 178	$ 457	$ 470
Income taxes at statutory rate	10	63	160	165
Effect of stock-based compensation	(1)	0	(14)	0
Effect of tax exempt interest	(5)	(5)	(17)	(18)
Effect of dividends received deduction	(2)	(1)	(6)	(5)
Effect of employee stock ownership plan dividends paid deduction	0	0	(2)	(1)
Effect of change in valuation allowance	16	7	16	40
Effect of subsidiaries not in AFG's tax return	0	2	0	4
Effect of other income tax reconciliation	0	(1)	9	5
Provision for income taxes as shown on the Statement of Earnings	$ 18	$ 65	$ 146	$ 190
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income taxes at statutory rate as a percentage of EBT	35.00%	35.00%	35.00%	35.00%
Effect of stock-based compensation as a percentage of EBT	(3.00%)	0.00%	(3.00%)	0.00%
Effect of tax exempt interest as a percentage of EBT	(17.00%)	(3.00%)	(4.00%)	(4.00%)
Effect of dividends received deduction as a percentage of EBT	(7.00%)	(1.00%)	(1.00%)	(1.00%)
Effect of employee stock ownership plan dividends paid deduction as a percentage of EBT	(0.00%)	(0.00%)	(0.00%)	(0.00%)
Effect of change in valuation allowance as a percentage of EBT	55.00%	4.00%	4.00%	9.00%
Effect of subsidiaries not in AFG's tax return as a percentage of EBT	0.00%	1.00%	0.00%	1.00%
Effect of other income tax reconciliation as a percentage of EBT	(1.00%)	1.00%	1.00%	0.00%
Provision for income taxes as shown on the Statement of Earnings as a percentage of EBT	62.00%	37.00%	32.00%	40.00%