Investments - Schedule of sources of net investment income (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net Investment Income [Line Items]
Investment income		$ 533	$ 475	$ 1,568	$ 1,379
Investment expenses		(6)	(4)	(16)	(13)
Net investment income		527	471	1,552	1,366
Fixed maturities
Net Investment Income [Line Items]
Investment income		440	405	1,283	1,191
Equity in earnings of partnerships and similar investments
Net Investment Income [Line Items]
Investment income		41	20	128	51
Other
Net Investment Income [Line Items]
Investment income		31	33	82	80
Accounting Standards Update 2016-01 | Equity securities
Net Investment Income [Line Items]
Investment income	[1]	2	0	16	4
Investment income | Equity securities
Net Investment Income [Line Items]
Investment income		$ 19	$ 17	$ 59	$ 53

[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.