Investments - Schedule of sources of net investment income (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net Investment Income [Line Items]
Gross investment income		$ 2,325	$ 2,116	$ 1,852
Investment expenses		(22)	(22)	(21)
Net investment income		2,303	2,094	1,831
Fixed maturities
Net Investment Income [Line Items]
Gross investment income		1,915	1,742	1,607
Equity in earnings of partnerships and similar investments
Net Investment Income [Line Items]
Gross investment income		154	161	64
Other
Net Investment Income [Line Items]
Gross investment income		132	112	102
Investment income | Equity securities
Net Investment Income [Line Items]
Gross investment income		85	79	73
Accounting Standards Update 2016-01 | Equity securities
Net Investment Income [Line Items]
Gross investment income	[1]	$ 39	$ 22	$ 6

[1] Although the change in the fair value of the majority of AFG’s equity securities is recorded in realized gains (losses) on securities, AFG records holding gains and losses in net investment income on equity securities classified as “trading” under previous guidance and on a small portfolio of limited partnership and similar investments that do not qualify for the equity method of accounting.