Insurance - Narrative (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended									12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Insurance [Line Items]
Carrying value of cash and securities owned by U.S.-based insurance subsidiaries on deposit		$ 1,040									$ 1,040
Carrying value of cash and securities owned by insurance subsidiaries on deposit in support of underwriting activities at Lloyd's		217									217
Undrawn letters of credit		414									414
Undrawn letters of credit, collateralized		0									$ 0
Workers' compensation insurance discount rate											4.50%	4.50%
Workers' compensation insurance discount, which has reduced the liability for unpaid losses and loss adjustment expenses		12				$ 13					$ 12	$ 13
Prior year claims and claims adjustment expense		45	$ 12	$ 41	$ 45	61	$ 31	$ 44	$ 56		143	192
Long-term care insurance reserves, net of reinsurance recoverables		46				45					46	45
Investment in Federal Home Loan Bank capital stock		52									52
Advances from Federal Home Loan Bank		1,100				1,100					$ 1,100	1,100
Proceeds from Federal Home Loan Bank Advance, Investing Activities						225
Description of variable rate basis											LIBOR
Repayments of advances from Federal Home Loan Bank due in next twelve months		310									$ 310
Repayments of advances from Federal Home Loan Bank due in year two		786									786
Fair value of fixed maturity investments held as collateral by the Federal Home Loan Bank		1,270									1,270
Interest on advances from Federal Home Loan Bank											27	20	$ 14
Maximum amount of dividends available to be paid by insurance subsidiaries to AFG without prior approval of regulatory authorities		852									852
Dividends on common stock											(446)	(397)	(421)
Aggregate guaranteed minimum death benefit value on variable annuity polices in force		13				35					13	35
Minimum
Insurance [Line Items]
Dividends payments without violating the most restrictive debt covenants, minimum		2,500									$ 2,500
Minimum | LIBOR
Insurance [Line Items]
FHLB advances, basis spread on variable rate											0.13%
Maximum | LIBOR
Insurance [Line Items]
FHLB advances, basis spread on variable rate											0.21%
Average | LIBOR
Insurance [Line Items]
FHLB advances, basis spread on variable rate											1.95%
FHLB, advances, refinanced terms
Insurance [Line Items]
Advances from Federal Home Loan Bank		610				40					$ 610	40
Neon exited lines charge
Insurance [Line Items]
Prior year claims and claims adjustment expense											$ 7	0	(18)
Catastrophe Bonds
Insurance [Line Items]
Catastrophe reinsurance coverage, excess retention percentage											95.00%
Catastrophe reinsurance coverage, excess retention amount											$ 200
Catastrophe reinsurance coverage, retention amount											134
Catastrophe reinsurance coverage, annual cost											11
Life insurance
Insurance [Line Items]
Increased statutory surplus resulting from reinsurance treaty		2,868				2,701					2,868	2,701
Life insurance in force, ceded premiums		6,230				7,690					6,230	7,690
Life insurance in force, direct premiums		$ 9,530				10,820					9,530	10,820
Reinsurance ceded											20	22	28
Reinsurance recoveries											32	38	35
Variable annuities | Maximum
Insurance [Line Items]
Death benefits paid in excess of the variable annuity account balances											1	1	1
Property and casualty insurance | Other lines
Insurance [Line Items]
Special A&E charges			$ 18				$ 18				$ 18	18	89
Neon Capital Limited | Property and casualty insurance
Insurance [Line Items]
Favorable reserve development related to certain lines of business within subsidiaries in connection with a reinsurance to close agreement										$ 42
Neon Capital Limited | Property and casualty insurance | Other lines
Insurance [Line Items]
Favorable reserve development related to certain lines of business within subsidiaries in connection with a reinsurance to close agreement										18			$ 18
Neon Capital Limited | Property and casualty insurance | Other lines | Reinsurance to close agreement
Insurance [Line Items]
Favorable reserve development related to certain lines of business within subsidiaries in connection with a reinsurance to close agreement	$ 18
Neon Capital Limited | Neon Capital Limited | Current lines
Insurance [Line Items]
Favorable reserve development related to certain lines of business within subsidiaries in connection with a reinsurance to close agreement										$ 24
Great American Life Insurance Company | Life insurance
Insurance [Line Items]
Increased statutory surplus resulting from reinsurance treaty						$ 510						$ 510