Investments - Schedule of sources of net investment income (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net Investment Income [Line Items]
Investment income		$ 535	$ 464	$ 1,035	$ 904
Investment expenses		(5)	(4)	(10)	(9)
Net investment income		530	460	1,025	895
Fixed maturities
Net Investment Income [Line Items]
Investment income		431	397	843	786
Equity in earnings of partnerships and similar investments
Net Investment Income [Line Items]
Investment income		41	21	87	31
Other
Net Investment Income [Line Items]
Investment income		28	27	51	47
Accounting Standards Update 2016-01 | Equity securities
Net Investment Income [Line Items]
Investment income	[1]	15	2	14	4
Investment income | Equity securities
Net Investment Income [Line Items]
Investment income		$ 20	$ 17	$ 40	$ 36

[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.