Investments - Schedule of sources of net investment income (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Net Investment Income [Line Items]
Investment income		$ 586	$ 535	$ 1,134	$ 1,035
Investment expenses		(6)	(5)	(12)	(10)
Net investment income		580	530	1,122	1,025
Fixed maturities
Net Investment Income [Line Items]
Investment income		478	431	947	843
Equity in earnings of partnerships and similar investments
Net Investment Income [Line Items]
Investment income		45	41	66	87
Other
Net Investment Income [Line Items]
Investment income		34	28	59	51
Accounting Standards Update 2016-01 | Equity securities
Net Investment Income [Line Items]
Investment income	[1]	7	15	18	14
Investment income | Equity securities
Net Investment Income [Line Items]
Investment income		$ 22	$ 20	$ 44	$ 40

[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.