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Quarterly Operating Results (Unaudited) - Narrative (Details) - USD ($) $ in Millions	3 Months Ended							12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Operating Results (Unaudited) [Line Items]
Pretax catastrophe losses		$ 107						$ 140	$ 55
Income tax expense (benefit)								247	119	$ 195
Property and casualty insurance | Other lines
Quarterly Operating Results (Unaudited) [Line Items]
Special charges to increase asbestos and environmental reserves		89				$ 36		89	36	67
Favorable reserve development related to certain lines of business within subsidiaries in connection with a reinsurance to close agreement								18
Special charges related to the exit of certain lines of business within subsidiaries							$ 65		65
Property and casualty insurance | Other lines | Increased insurance loss reserves related to medical malpractice and general liability lines
Quarterly Operating Results (Unaudited) [Line Items]
Special charges related to the exit of certain lines of business within subsidiaries							57
Other
Quarterly Operating Results (Unaudited) [Line Items]
Special charges to increase asbestos and environmental reserves								24	5	12
Write off of deferred debt issuance cost due to the retirement of debt		4	$ 7							4
Make-whole premium, settlement of interest rate swap and write off of deferred debt issuance cost included in the loss on the retirement of debt	$ 40							51
Other | Former railroad and manufacturing operations
Quarterly Operating Results (Unaudited) [Line Items]
Special charges to increase asbestos and environmental reserves		$ 24				$ 5
Neon Capital Limited
Quarterly Operating Results (Unaudited) [Line Items]
Income tax expense (benefit)	(56)				$ (111)				(48)
Neon Capital Limited | Property and casualty insurance
Quarterly Operating Results (Unaudited) [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
Quarterly Operating Results (Unaudited) [Line Items]
Favorable reserve development related to certain lines of business within subsidiaries in connection with a reinsurance to close agreement	18
Special charges related to the exit of certain lines of business within subsidiaries									65
Neon Capital Limited | Neon Capital Limited | Current lines
Quarterly Operating Results (Unaudited) [Line Items]
Favorable reserve development related to certain lines of business within subsidiaries in connection with a reinsurance to close agreement	24
AFG
Quarterly Operating Results (Unaudited) [Line Items]
Income tax expense (benefit)	$ (56)				$ (177)			$ 247	119	195
Real estate investment | Property and casualty insurance
Quarterly Operating Results (Unaudited) [Line Items]
Pretax income from the sale of real estate properties included in other income				$ 13			$ 32		$ 32	$ 66

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This element represents the amount of gain (loss) on sale of properties during the reporting period gross of the applicable income taxes realized.

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-Publisher FASB
-Name Accounting Standards Codification
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