Investments - Narrative (Details) $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2020 USD ($) security		Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) security		Jun. 30, 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						86.00%
Net investment income		$ 468	[1]		$ 580	$ 1,012	[1]	$ 1,122
Investment income		474			586	1,023		1,134
Fixed maturities
Schedule of Investments [Line Items]
Gross unrealized losses on fixed maturities		457				$ 457			$ 88
Number of available for sale securities in an unrealized loss position | security						993
Residential MBS
Schedule of Investments [Line Items]
Gross unrealized losses on fixed maturities		$ 12				$ 12			5
Number of securities purchased with credit deterioration | security		2
Aggregate par value of securities purchased with credit deterioration, at time of purchase		$ 8
Aggregate purchase price of securities purchased with credit deterioration, at time of purchase		6
Initial allowance for purchased securities with credit deterioration		1
Initial discount for purchased securities with credit deterioration		1
Collateralized loan obligations
Schedule of Investments [Line Items]
Gross unrealized losses on fixed maturities		114				114			37
Other asset-backed securities
Schedule of Investments [Line Items]
Gross unrealized losses on fixed maturities		189				189			$ 20
Neon Capital Limited
Schedule of Investments [Line Items]
Net investment income						(6)
Neon Capital Limited | Maximum
Schedule of Investments [Line Items]
Net investment income		1
Accounting Standards Update 2016-01 | Equity securities
Schedule of Investments [Line Items]
Investment income	[2]	$ 9	[1]		$ 7	$ (3)	[1]	$ 18
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 second quarter and first six months of 2020 includes income of less than $1 million and losses of $6 million, respectively, on investments held by the companies that comprise the Neon exited lines due primarily to the $7 million loss in first quarter of 2020 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.