Acquisitions and Sale of Businesses - Narrative on acquisitions of subsidiaries (Details) - USD ($) $ / shares in Units, $ in Millions			1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Nov. 10, 2016	Apr. 01, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Business Acquisitions [Line Items]
Payments to acquire additional interest in subsidiaries						$ 315	$ 0	$ 0
Income tax expense (benefit)						119	195	220
Payments to acquire business						$ 0	0	267
Weighted average useful life of finite-lived intangible assets acquired						4 years
Goodwill				$ 199	$ 201	$ 199	199	201	$ 185
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
Summit Holding Southeast, Inc
Business Acquisitions [Line Items]
Payments to acquire business		$ 259
Capital contributions to acquired business		140
Total capital investment in acquired business		399
Net earned premiums of acquired business					$ 410	540	529
Goodwill		16
Selective Insurance Company of America Self-Insured Pooled Group
Business Acquisitions [Line Items]
Payments to acquire business			$ 8
In-force gross written premiums of acquired book of business			$ 38
Gross written premiums of acquired business						34
Net written premiums of acquired business						16
Maximum | Summit Holding Southeast, Inc
Business Acquisitions [Line Items]
Payments for acquisition related costs		$ 1
AFG
Business Acquisitions [Line Items]
Income tax expense (benefit)				(177)		119	195	220
Capital contributions to acquired business						$ 560	$ 27	$ 431
AFG | National Interstate
Business Acquisitions [Line Items]
Reduction to capital surplus	$ (137)
Income tax expense (benefit)				$ (66)