Insurance - Reinsurance Retention (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($)
California Workers’ Compensation
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, amount retained	$ 2
Reinsurance coverage amount	$ 148
Reinsurance coverage amount by internal reinsurance program as a percentage of reinsurance coverage amount	1.00%	[1]
Reinsurance retention policy, maximum loss for claims up to the reinsurance coverage limit	$ 3	[2]
Other Workers’ Compensation
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, amount retained	3
Reinsurance coverage amount	$ 37
Reinsurance coverage amount by internal reinsurance program as a percentage of reinsurance coverage amount	0.00%	[1]
Reinsurance retention policy, maximum loss for claims up to the reinsurance coverage limit	$ 3	[2]
Commercial Umbrella
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, amount retained	1
Reinsurance coverage amount	$ 49
Reinsurance coverage amount by internal reinsurance program as a percentage of reinsurance coverage amount	13.00%	[1]
Reinsurance retention policy, maximum loss for claims up to the reinsurance coverage limit	$ 7	[2]
Property — General
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, amount retained	5
Reinsurance coverage amount	$ 45
Reinsurance coverage amount by internal reinsurance program as a percentage of reinsurance coverage amount	0.00%	[1]
Reinsurance retention policy, maximum loss for claims up to the reinsurance coverage limit	$ 5	[2]
Property — Catastrophe
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, amount retained	15
Reinsurance coverage amount	$ 85
Reinsurance coverage amount by internal reinsurance program as a percentage of reinsurance coverage amount	5.00%	[1]
Reinsurance retention policy, maximum loss for claims up to the reinsurance coverage limit	$ 19	[2]
Neon Lloyd’s Syndicate
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, amount retained	15
Reinsurance coverage amount	$ 210
Reinsurance coverage amount by internal reinsurance program as a percentage of reinsurance coverage amount	0.00%	[1]
Reinsurance retention policy, maximum loss for claims up to the reinsurance coverage limit	$ 15	[2]
Riverfront Re Ltd. catastrophe bond
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, amount retained	100	[3]
Reinsurance coverage amount	$ 200	[3]
Reinsurance coverage amount by internal reinsurance program as a percentage of reinsurance coverage amount	5.00%	[1],[3]
AFG | California Workers’ Compensation
Reinsurance Retention Policy [Line Items]
Reinsurance coverage amount	$ 1	[1]
AFG | Other Workers’ Compensation
Reinsurance Retention Policy [Line Items]
Reinsurance coverage amount	0	[1]
AFG | Commercial Umbrella
Reinsurance Retention Policy [Line Items]
Reinsurance coverage amount	6	[1]
AFG | Property — General
Reinsurance Retention Policy [Line Items]
Reinsurance coverage amount	0	[1]
AFG | Property — Catastrophe
Reinsurance Retention Policy [Line Items]
Reinsurance coverage amount	4	[1]
AFG | Neon Lloyd’s Syndicate
Reinsurance Retention Policy [Line Items]
Reinsurance coverage amount	0	[1]
AFG | Riverfront Re Ltd. catastrophe bond
Reinsurance Retention Policy [Line Items]
Reinsurance coverage amount	$ 10	[1],[3]

[1] Includes the participation of AFG’s internal reinsurance program.

[2] Maximum loss for claims up to reinsurance coverage limit.

[3] Includes aggregate coverage. See description below.