Acquisitions and Sale of Businesses - Narrative on acquisitions of subsidiaries (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	3 Months Ended			12 Months Ended
	Nov. 10, 2016	Nov. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisitions [Line Items]
Payments to acquire subsidiaries						$ 36	$ 0	$ 0
Goodwill acquired during period			$ 8			8
Payments to acquire additional interest in subsidiaries						0	0	315
Income tax expense (benefit)						122	247	119
National Interstate
Business Acquisitions [Line Items]
Percentage ownership of National Interstate by noncontrolling owners prior to acquisition	49.00%
Payments to acquire additional interest in subsidiaries	$ 315
Acquisition price per share (USD per share)	$ 32.00
Cash dividend paid (USD per share)	$ 0.50
Payment of special cash dividends	$ 5
Payments for acquisition related costs					$ 10
ABA Insurance Services
Business Acquisitions [Line Items]
Payments to acquire subsidiaries		$ 30
Contingent consideration			$ 3			$ 3
Contingent consideration term						4 years
Finite-lived intangible assets acquired		25
Finite-lived intangible asset useful life						9 years
Goodwill acquired during period		$ 8
AFG
Business Acquisitions [Line Items]
Income tax expense (benefit)				$ (56)		$ 122	$ 247	$ 119
AFG | National Interstate
Business Acquisitions [Line Items]
Reduction to capital surplus	$ (137)
Income tax expense (benefit)					$ (66)