Acquisitions and Sale of Businesses - Narrative on sale of businesses (Details) $ in Millions				3 Months Ended					12 Months Ended
		Dec. 24, 2015 USD ($)	Mar. 31, 2015	Dec. 31, 2017 USD ($) policy	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) policy	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Nov. 30, 2015
Sale of Businesses
Sale of redeemable noncontrolling interests									$ (3)
Income tax expense (benefit)									247	$ 119	$ 195
Long-term care insurance reserves				$ 39	$ 37				39	37
Realized gains (losses) on subsidiaries									0	2	(161)
Property and Casualty Group
Sale of Businesses
Realized gains (losses) on subsidiaries							$ 5
Neon Capital Limited
Sale of Businesses
Sale of redeemable noncontrolling interests				(3)
Income tax expense (benefit)				(56)	(111)					(48)
AFG
Sale of Businesses
Income tax expense (benefit)				$ (56)	$ (177)				247	119	$ 195
Run-off long-term care insurance business
Sale of Businesses
Net sale proceeds	[1]	$ 13
Potential additional sale proceeds based on the release of certain statutory liabilities of the legal entities sold									$ 13
Contingent capital support agreement amount		$ 35
Contingent capital support agreement term		5 years
Long-term care insurance reserves of business sold as a percentage of total long-term care insurance reserves												96.00%
Total tax benefits proceeds from the sale of business										$ 97
Realized gains (losses) on subsidiaries		$ (166)				$ 2		$ (162)
Loss on subsidiaries, net of tax		$ (108)						$ (105)
Carrying value of the assets and liabilities of business sold as a percentage of total assets and liabilities		4.00%	4.00%
Run-off long-term care insurance business | AFG
Sale of Businesses
Number of long-term care insurance policies | policy				1,600					1,600
Long-term care insurance reserves				$ 39					$ 39

[1] Includes the fair value of the potential additional consideration and capital maintenance agreement and is shown net of estimated expenses.