Investments - Schedule of sources of net investment income (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Investment Income [Line Items]
Gross investment income		$ 2,116	$ 1,852	$ 1,716
Investment expenses		(22)	(21)	(20)
Net investment income		2,094	1,831	1,696
Fixed maturities
Net Investment Income [Line Items]
Gross investment income		1,742	1,607	1,510
Equity in earnings of partnerships and similar investments
Net Investment Income [Line Items]
Gross investment income		161	64	44
Other
Net Investment Income [Line Items]
Gross investment income		112	102	81
Investment income | Equity securities
Net Investment Income [Line Items]
Gross investment income		79	73	77
Accounting Standards Update 2016-01 | Equity securities
Net Investment Income [Line Items]
Gross investment income	[1]	$ 22	$ 6	$ 4

[1] As discussed in Note A — “Accounting Policies — Investments,” AFG adopted guidance in January 2018 that requires all equity securities other than those accounted for under the equity method to be reported at fair value with holding gains and losses recognized in net earnings. 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 the previous guidance and on a small portfolio of limited partnership and similar investments that do not qualify for the equity method of accounting.