Investments - Narrative (Details) $ in Millions		3 Months Ended
		Mar. 31, 2020 USD ($) security		Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Schedule of Investments [Line Items]
Percentage (based on amount of unrealized loss) of available for sale fixed maturities that are in an unrealized loss position and rated investment grade		83.00%
Percentage (based on fair value) of available for sale fixed maturities that are in an unrealized loss position and rated investment grade		89.00%
Allowance for credit losses		$ 61
Net investment income		544	[1]	$ 542
Investment income		549		548
Fixed maturities
Schedule of Investments [Line Items]
Gross unrealized losses on fixed maturities		$ (1,324)			$ (88)
Number of available for sale securities in an unrealized loss position | security		1,871
Residential MBS
Schedule of Investments [Line Items]
Gross unrealized losses on fixed maturities		$ (74)			(5)
Allowance for credit losses		6
Collateralized loan obligations
Schedule of Investments [Line Items]
Gross unrealized losses on fixed maturities		(306)			(37)
Allowance for credit losses		17
Corporate bonds
Schedule of Investments [Line Items]
Allowance for credit losses		24
Other asset-backed securities
Schedule of Investments [Line Items]
Gross unrealized losses on fixed maturities		(362)			$ (20)
Allowance for credit losses		14
Neon Capital Limited
Schedule of Investments [Line Items]
Net investment income		(6)
Accounting Standards Update 2016-01 | Equity securities
Schedule of Investments [Line Items]
Investment income	[2]	(12)	[1]	$ 11
Accounting Standards Update 2016-01 | Equity securities | Neon Capital Limited
Schedule of Investments [Line Items]
Investment income	[2]	$ (7)

[1] Net investment income in the first three months of 2020 includes a loss of $6 million on investments held by the companies that comprise the Neon exited lines due primarily to the $7 million loss recorded on equity securities that are carried at fair value through net investment income.

[2] 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.